Added Dart tutorial.

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+---
+language: Dart
+author: Joao Pedrosa
+author_url: github.com/jpedrosa/
+---
+
+Dart is a nwecomer into the realm of programming languages.
+It borrows a lot from other mainstream languages, having as a goal not to deviate too much from
+its JavaScript sibling. Like JavaScript, Dart aims for great browser integration.
+
+Dart's most controversial feature must be its Optional Typing.
+
+```
+import "dart:collection";
+import "dart:math" as DM;
+
+// Welcome to Learn Dart in 15 minutes. http://www.dartlang.org/
+// This is an executable tutorial. You can run it with Dart or on
+// the Try Dart! site if you copy/paste it there. http://try.dartlang.org/
+
+// Function declaration and method declaration look the same. Function
+// declarations can be nested. The declaration takes the form of
+// name() {} or name() => singleLineExpression;
+// The fat arrow function declaration has an implicit return for the result of
+// the expression.
+example1() {
+  example1nested1() {
+    example1nested2() => print("Example1 nested 1 nested 2");
+    example1nested2();
+  }
+  example1nested1();
+}
+
+// Anonymous functions don't include a name.
+example2() {
+  example2nested1(fn) {
+    fn();
+  }
+  example2nested1(() => print("Example2 nested 1"));
+}
+
+// When a function parameter is declared, the declaration can include the
+// number of parameters the function takes by specifying the names of the
+// parameters it takes.
+example3() {
+  example3nested1(fn(informSomething)) {
+    fn("Example3 nested 1");
+  }
+  example3planB(fn) { // Or don't declare number of parameters.
+    fn("Example3 plan B");
+  }
+  example3nested1((s) => print(s));
+  example3planB((s) => print(s));
+}
+
+// Functions have closure access to outer variables.
+var example4Something = "Example4 nested 1";
+example4() {
+  example4nested1(fn(informSomething)) {
+    fn(example4Something);
+  }
+  example4nested1((s) => print(s));
+}
+
+// Class declaration with a sayIt method, which also has closure access
+// to the outer variable as though it were a function as seen before.
+var example5method = "Example5 sayIt";
+class Example5Class {
+  sayIt() {
+    print(example5method);
+  }
+}
+example5() {
+  // Create an anonymous instance of the Example5Class and call the sayIt
+  // method on it.
+  new Example5Class().sayIt();
+}
+
+// Class declaration takes the form of class name { [classBody] }.
+// Where classBody can include instance methods and variables, but also
+// class methods and variables.
+class Example6Class {
+  var example6InstanceVariable = "Example6 instance variable"; 
+  sayIt() {
+    print(example6InstanceVariable);
+  }
+}
+example6() {
+  new Example6Class().sayIt();
+}
+
+// Class methods are variables are declared with "static" terms.
+class Example7Class {
+  static var example7ClassVariable = "Example7 class variable"; 
+  static sayItFromClass() {
+    print(example7ClassVariable);
+  }
+  sayItFromInstance() {
+    print(example7ClassVariable);
+  }
+}
+example7() {
+  Example7Class.sayItFromClass();
+  new Example7Class().sayItFromInstance();
+}
+
+// Literals are great, but there's a restriction for what literals can be
+// outside of function/method bodies. Literals on the outer scope of class
+// or outside of class have to be constant. Strings and numbers are constant
+// by default. But arrays and maps are not. They can be made constant by
+// declaring them "const".
+var example8A = const ["Example8 const array"],
+  example8M = const {"someKey": "Example8 const map"}; 
+example8() {
+  print(example8A[0]);
+  print(example8M["someKey"]);
+}
+
+// Loops in Dart take the form of standard for () {} or while () {} loops,
+// slightly more modern for (.. in ..) {}, or functional callbacks with many
+// supported features, starting with forEach.
+var example9A = const ["a", "b"];
+example9() {
+  for (var i = 0; i < example9A.length; i++) {
+    print("Example9 for loop '${example9A[i]}'");
+  }
+  var i = 0;
+  while (i < example9A.length) {
+    print("Example9 while loop '${example9A[i]}'");
+    i++;
+  }
+  for (var e in example9A) {
+    print("Example9 for-in loop '${e}'");
+  }
+  example9A.forEach((e) => print("Example9 forEach loop '${e}'"));
+}
+
+// To loop over the characters of a string or to extract a substring.
+var example10S = "ab";
+example10() {
+  for (var i = 0; i < example10S.length; i++) {
+    print("Example10 String character loop '${example10S[i]}'");
+  }
+  for (var i = 0; i < example10S.length; i++) {
+    print("Example10 substring loop '${example10S.substring(i, i + 1)}'");
+  }
+}
+
+// Int and double are the two supported number formats.
+example11() {
+  var i = 1 + 320, d = 3.2 + 0.01;
+  print("Example11 int ${i}");
+  print("Example11 double ${d}");
+}
+
+// DateTime provides date/time arithmetic.
+example12() {
+  var now = new DateTime.now();
+  print("Example12 now '${now}'");
+  now = now.add(new Duration(days: 1));
+  print("Example12 tomorrow '${now}'");
+}
+
+// Regular expressions are supported.
+example13() {
+  var s1 = "some string", s2 = "some", re = new RegExp("^s.+?g\$");
+  match(s) {
+    if (re.hasMatch(s)) {
+      print("Example13 regexp matches '${s}'");
+    } else {
+      print("Example13 regexp doesn't match '${s}'");
+    }
+  }
+  match(s1);
+  match(s2);
+}
+
+// Boolean expressions need to resolve to either true or false, as no
+// implicit conversions are supported.
+example14() {
+  var v = true;
+  if (v) {
+    print("Example14 value is true");
+  }
+  v = null;
+  try {
+    if (v) {
+      // Never runs
+    } else {
+      // Never runs
+    }
+  } catch (e) {
+    print("Example14 null value causes an exception: '${e}'");
+  }
+}
+
+// try/catch/finally and throw are used for exception handling.
+// throw takes any object as parameter;
+example15() {
+  try {
+    try {
+      throw "Some unexpected error.";
+    } catch (e) {
+      print("Example15 null value causes an exception: '${e}'");
+      throw e; // Re-throw
+    }
+  } catch (e) {
+    print("Example15 catch exception being re-thrown: '${e}'");
+  } finally {
+    print("Example15 Still run finally");
+  }
+}
+
+// To be efficient when creating a long string dynamically, use
+// StringBuffer. Or you could join a string array.
+example16() {
+  var sb = new StringBuffer(), a = ["a", "b", "c", "d"], e;
+  for (e in a) { sb.write(e); }
+  print("Example16 dynamic string created with "
+    "StringBuffer '${sb.toString()}'");
+  print("Example16 join string array '${a.join()}'");
+}
+
+// Strings can be concatenated by just having string literals next to
+// one another with no further operator needed.
+example17() {
+  print("Example17 "
+      "concatenate "
+      "strings "
+      "just like that");
+}
+
+// Strings have single-quote or double-quote for delimiters with no
+// actual difference between the two. The given flexibility can be good
+// to avoid the need to escape content that matches the delimiter being
+// used. For example, double-quotes of HTML attributes if the string
+// contains HTML content.
+example18() {
+  print('Example18 <a href="etc">'
+      "Don't can't I'm Etc"
+      '</a>');
+}
+
+// Strings with triple single-quotes or triple double-quotes span
+// multiple lines and include line delimiters.
+example19() {
+  print('''Example19 <a href="etc"> 
+Example19 Don't can't I'm Etc
+Example19 </a>''');
+}
+
+// Strings have the nice interpolation feature with the $ character.
+// With $ { [expression] }, the return of the expression is interpolated.
+// $ followed by a variable name interpolates the content of that variable.
+// $ can be escaped like so \$ to just add it to the string instead.
+example20() {
+  var s1 = "'\${s}'", s2 = "'\$s'";
+  print("Example20 \$ interpolation ${s1} or $s2 works.");
+}
+
+// Optional types allow for the annotation of APIs and come to the aid of
+// IDEs so the IDEs can better refactor, auto-complete and check for
+// errors. So far we haven't declared any types and the programs have
+// worked just fine. In fact, types are disregarded during runtime.
+// Types can even be wrong and the program will still be given the
+// benefit of the doubt and be run as though the types didn't matter.
+// There's a runtime parameter that checks for type errors which is
+// the checked mode, which is said to be useful during development time,
+// but which is also slower because of the extra checking and is thus
+// avoided during deployment runtime.
+class Example21 {
+  List<String> _names;
+  Example21() {
+    _names = ["a", "b"]; 
+  }
+  List<String> get names => _names;
+  set names(List<String> list) {
+    _names = list;
+  }
+  int get length => _names.length;
+  void add(String name) {
+    _names.add(name);
+  }
+}
+void example21() {
+  Example21 o = new Example21();
+  o.add("c");
+  print("Example21 names '${o.names}' and length '${o.length}'");
+  o.names = ["d", "e"];
+  print("Example21 names '${o.names}' and length '${o.length}'");
+}
+
+// Class inheritance takes the form of class name extends AnotherClassName {}.
+class Example22A {
+  var _name = "Some Name!";
+  get name => _name;
+}
+class Example22B extends Example22A {}
+example22() {
+  var o = new Example22B();
+  print("Example22 class inheritance '${o.name}'");
+}
+
+// Class mixin is also available, and takes the form of
+// class name extends SomeClass with AnotherClassName {}.
+// It's necessary to extend some class to be able to mixin another one.
+// The template class of mixin cannot at the moment have a constructor.
+// Mixin is mostly used to share methods with distant classes, so the
+// single inheritance doesn't get in the way of reusable code.
+// Mixins follow the "with" statement during the class declaration.
+class Example23A {}
+class Example23Utils {
+  addTwo(n1, n2) {
+    return n1 + n2;
+  }
+}
+class Example23B extends Example23A with Example23Utils {
+  addThree(n1, n2, n3) {
+    return addTwo(n1, n2) + n3;
+  }
+}
+example23() {
+  var o = new Example23B(), r1 = o.addThree(1, 2, 3),
+    r2 = o.addTwo(1, 2);
+  print("Example23 addThree(1, 2, 3) results in '${r1}'");
+  print("Example23 addTwo(1, 2) results in '${r2}'");
+}
+
+// The Class constructor method uses the same name of the class and
+// takes the form of SomeClass() : super() {}, where the ": super()"
+// part is optional and it's used to delegate constant parameters to the
+// super-parent's constructor.
+class Example24A {
+  var _value;
+  Example24A({value: "someValue"}) {
+    _value = value;
+  }
+  get value => _value;
+}
+class Example24B extends Example24A {
+  Example24B({value: "someOtherValue"}) : super(value: value);
+}
+example24() {
+  var o1 = new Example24B(),
+    o2 = new Example24B(value: "evenMore");
+  print("Example24 calling super during constructor '${o1.value}'");
+  print("Example24 calling super during constructor '${o2.value}'");
+}
+
+// There's a shortcut to set constructor parameters in case of simpler classes.
+// Just use the this.parameterName prefix and it will set the parameter on
+// an instance variable of same name.
+class Example25 {
+  var value, anotherValue;
+  Example25({this.value, this.anotherValue});
+}
+example25() {
+  var o = new Example25(value: "a", anotherValue: "b");
+  print("Example25 shortcut for constructor '${o.value}' and "
+    "'${o.anotherValue}'");
+}
+
+// Named parameters are available when declared between {}.
+// Parameter order can be optional when declared between {}.
+// Parameters can be made optional when declared between [].
+example26() {
+  var _name, _surname, _email;
+  setConfig1({name, surname}) {
+    _name = name;
+    _surname = surname;
+  }
+  setConfig2(name, [surname, email]) {
+    _name = name;
+    _surname = surname;
+    _email = email;
+  }
+  setConfig1(surname: "Doe", name: "John");
+  print("Example26 name '${_name}', surname '${_surname}', "
+    "email '${_email}'");
+  setConfig2("Mary", "Jane");
+  print("Example26 name '${_name}', surname '${_surname}', "
+  "email '${_email}'");
+}
+
+// Variables declared with final can only be set once.
+// In case of classes, final instance variables can be set via constant
+// constructor parameter.
+class Example27 {
+  final color1, color2;
+  // A little flexibility to set final instance variables with syntax
+  // that follows the :
+  Example27({this.color1, color2}) : color2 = color2;
+}
+example27() {
+  final color = "orange", o = new Example27(color1: "lilac", color2: "white");
+  print("Example27 color is '${color}'");
+  print("Example27 color is '${o.color1}' and '${o.color2}'");
+}
+
+// To import a library, use import "libraryPath" or if it's a core library,
+// import "dart:libraryName". There's also the "pub" package management with
+// its own convention of import "package:packageName".
+// See import "dart:collection"; at the top. Imports must come before
+// other code declarations. IterableBase comes from dart:collection.
+class Example28 extends IterableBase {
+  var names;
+  Example28() {
+    names = ["a", "b"];
+  }
+  get iterator => names.iterator;
+}
+example28() {
+  var o = new Example28();
+  o.forEach((name) => print("Example28 '${name}'"));
+}
+
+// For control flow we have:
+// * standard switch with must break statements
+// * if-else if-else and ternary ..?..:.. operator
+// * closures and anonymous functions
+// * break, continue and return statements
+example29() {
+  var v = true ? 30 : 60;
+  switch (v) {
+    case 30:
+      print("Example29 switch statement");
+      break;
+  }
+  if (v < 30) {
+  } else if (v > 30) {
+  } else {
+    print("Example29 if-else statement");
+  }
+  callItForMe(fn()) {
+    return fn();
+  }
+  rand() {
+    v = new DM.Random().nextInt(50);
+    return v;
+  }
+  while (true) {
+    print("Example29 callItForMe(rand) '${callItForMe(rand)}'");
+    if (v != 30) {
+      break;
+    } else {
+      continue;
+    }
+    // Never gets here.
+  }
+}
+
+// Parse int, convert double to int, or just keep int when dividing numbers
+// by using the ~/ operation. Let's play a guess game too.
+example30() {
+  var gn, tooHigh = false,
+    n, n2 = (2.0).toInt(), top = int.parse("123") ~/ n2, bottom = 0;
+  top = top ~/ 6;
+  gn = new DM.Random().nextInt(top + 1); // +1 because nextInt top is exclusive
+  print("Example30 Guess a number between 0 and ${top}");
+  guessNumber(i) {
+    if (n == gn) {
+      print("Example30 Guessed right! The number is ${gn}");
+    } else {
+      tooHigh = n > gn;
+      print("Example30 Number ${n} is too "
+        "${tooHigh ? 'high' : 'low'}. Try again");
+    }
+    return n == gn;
+  }
+  n = (top - bottom) ~/ 2;
+  while (!guessNumber(n)) {
+    if (tooHigh) {
+      top = n - 1;
+    } else {
+      bottom = n + 1;
+    }
+    n = bottom + ((top - bottom) ~/ 2);
+  }
+}
+
+// Programs have only one entry point in the main function.
+// Nothing is expected to be executed on the outer scope before a program
+// starts running with what's in its main function.
+// This helps with faster loading and even lazily loading of just what
+// the program needs to startup with.
+main() {
+  print("Learn Dart in 15 minutes!");
+  [example1, example2, example3, example4, example5, example6, example7,
+    example8, example9, example10, example11, example12, example13, example14,
+    example15, example16, example17, example18, example19, example20,
+    example21, example22, example23, example24, example25, example26,
+    example27, example28, example29, example30
+    ].forEach((ef) => ef());
+}
+
+```
+
+## Further Reading
+
+Dart has a compreenshive web-site. It covers API reference, tutorials, articles and more, including a
+useful Try Dart online.
+http://www.dartlang.org/
+http://try.dartlang.org/
+
+
+