Make mappedBy lazy.

sdk/lib/html/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 14 matching lines @@
   }
 
 $if DEFINE_CONTAINS
   bool contains($E element) => _Collections.contains(this, element);
 $else
   // contains() defined by IDL.
 $endif
 
   void forEach(void f($E element)) => _Collections.forEach(this, f);
 
-  Collection mappedBy(f($E element)) => _Collections.mappedBy(this, [], f);
+  Iterable mappedBy(f($E element)) =>
+      new MappedIterable<$E, dynamic>(this, f);
 
   Collection<$E> where(bool f($E element)) =>
      _Collections.where(this, <$E>[], f);
 
   bool every(bool f($E element)) => _Collections.every(this, f);
 
   bool some(bool f($E element)) => _Collections.some(this, f);
 
   bool get isEmpty => this.length == 0;
 
@@ skipping 26 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.