Make 'where' 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 2f6115715f6284bbc520a8f13475ce1e91763c86..25f39e01628e06bc93911843cee2d50cd4d15c76 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
@@ -473,11 +473,8 @@ class NodeList extends Node implements Iterable<Node> {
   // mechanism.
   Iterable mappedBy(f(Node element)) => new MappedIterable(this, f);
 
-  Collection<Node> where(bool f(Node element)) {
-    List result = <Node>[];
-    for (Node element in this) if (f(element)) result.add(element);
-    return result;
-  }
+  Iterable<Node> where(bool f(Node element))
+      => new FilteredIterable<Node>(this, f);
 
   bool contains(Node element) {
     for (Node e in this) {