Add example int doc for Iterable.expand

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 3e7c1cbd97a3327e64d05cff823dcfb37358c120..52476bafc0ca9244084e31fb7ef879137da48cad 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -181,6 +181,15 @@ abstract class Iterable<E> {
    *
    * The returned [Iterable] is lazy, and calls [f] for each element
    * of this every time it's iterated.
+   *
+   * Example:
+   *
+   *     var lists = [<int>[1, 2, 3], <int>[3, 4, 5]];
+   *     Set<int> ints = lists.expand((list) => list).toSet();
+   *     print(ints);

[floitsch, 2016/08/05 09:38:40]

I prefer not to write the type arguments and sets:

var pairs = [[1, 2], [3, 4]];
var flattened = pairs.expand((pair) => pair).toList();
print(flattened);  // => [1, 2, 3, 4];

var input = [1, 2, 3];
var duplicated = input.expand((i) => [i, i]).toList();
print(duplicated);  // => [1, 1, 2, 2, 3, 3]

[stanm, 2016/08/05 16:48:31]

Done.

+   *
+   *     > {1, 2, 3, 4, 5}
+   *
    */
   Iterable/*<T>*/ expand/*<T>*/(Iterable/*<T>*/ f(E element)) =>
       new ExpandIterable<E, dynamic/*=T*/>(this, f);