Make mappedBy lazy.

sdk/lib/_internal/compiler/implementation/tree/nodes.dart

Index: sdk/lib/_internal/compiler/implementation/tree/nodes.dart
diff --git a/sdk/lib/_internal/compiler/implementation/tree/nodes.dart b/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
index 3e241bfce754e9a0192c39dbc9c57a81c3443264..2f6115715f6284bbc520a8f13475ce1e91763c86 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
@@ -471,11 +471,7 @@ class NodeList extends Node implements Iterable<Node> {
   //
   // TODO(floitsch): these functions should be pulled in through a mixin
   // mechanism.
-  Collection mappedBy(f(Node element)) {
-    List result = [];
-    for (Node element in this) result.add(f(element));
-    return result;
-  }
+  Iterable mappedBy(f(Node element)) => new MappedIterable(this, f);
 
   Collection<Node> where(bool f(Node element)) {
     List result = <Node>[];