Added expand method to iterables.

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new FixedSizeListIterator<$E>(this);
@@ skipping 20 matching lines @@
 
   Iterable map(f($E element)) =>
       IterableMixinWorkaround.mapList(this, f);
 
   List mappedBy(f($E element)) =>
       IterableMixinWorkaround.mappedByList(this, f);
 
   Iterable<$E> where(bool f($E element)) =>
       IterableMixinWorkaround.where(this, f);
 
+  Iterable expand(Iterable f($E element)) =>
+      IterableMixinWorkaround.expand(this, f);
+
   bool every(bool f($E element)) => IterableMixinWorkaround.every(this, f);
 
   bool any(bool f($E element)) => IterableMixinWorkaround.any(this, f);
 
   List<$E> toList() => new List<$E>.from(this);
   Set<$E> toSet() => new Set<$E>.from(this);
 
   bool get isEmpty => this.length == 0;
 
   Iterable<$E> take(int n) => IterableMixinWorkaround.takeList(this, n);
@@ skipping 128 matching lines @@
   }
 
   void insertRange(int start, int rangeLength, [$E initialValue]) {
     throw new UnsupportedError("Cannot insertRange on immutable List.");
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       Lists.getRange(this, start, rangeLength, <$E>[]);
 
   // -- end List<$E> mixins.