Added expand method to iterables.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 3b457faa494fa798cade62fa09c65d31c9dd88dc..3ce453bba5e3625287e521949cf1ca9908b2a019 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -116,6 +116,10 @@ class _ObjectArray<E> implements List<E> {
     return IterableMixinWorkaround.where(this, f);
   }
 
+  Iterable expand(Iterable f(E element)) {
+    return IterableMixinWorkaround.expand(this, f);
+  }
+
   Iterable<E> take(int n) {
     return IterableMixinWorkaround.takeList(this, n);
   }
@@ -350,6 +354,10 @@ class _ImmutableArray<E> implements List<E> {
     return IterableMixinWorkaround.where(this, f);
   }
 
+  Iterable expand(Iterable f(E element)) {
+    return IterableMixinWorkaround.expand(this, f);
+  }
+
   Iterable<E> take(int n) {
     return IterableMixinWorkaround.takeList(this, n);
   }