dart2js: fix dependency tracking for deferred loading when types are only used

pkg/compiler/lib/src/resolution/tree_elements.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of resolution;
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
   /// Iterables of the dependencies that this [TreeElement] records of
   /// [analyzedElement].
   Iterable<Element> get allElements;
+
   void forEachConstantNode(f(Node n, ConstantExpression c));
+  void forEachType(f(Node n, DartType t));
 
   /// A set of additional dependencies.  See [registerDependency] below.
   Iterable<Element> get otherDependencies;
 
   Element operator[](Node node);
 
   SendStructure getSendStructure(Send send);
 
   // TODO(johnniwinther): Investigate whether [Node] could be a [Send].
   Selector getSelector(Node node);
@@ skipping 141 matching lines @@
 
   void setType(Node node, DartType type) {
     if (_types == null) {
       _types = new Maplet<Node, DartType>();
     }
     _types[node] = type;
   }
 
   DartType getType(Node node) => _types != null ? _types[node] : null;
 
+  void forEachType(f(Node n, DartType t)) {
+    if (_types != null) {
+      _types.forEach(f);
+    }
+  }
+
   Iterable<Node> get superUses {
     return _superUses != null ? _superUses : const <Node>[];
   }
 
   void addSuperUse(Node node) {
     if (_superUses == null) {
       _superUses = new Setlet<Node>();
     }
     _superUses.add(node);
   }
@@ skipping 340 matching lines @@
   TreeElements get treeElements {
     assert(invariant(this, _treeElements !=null,
         message: "TreeElements have not been computed for $this."));
     return _treeElements;
   }
 
   void reuseElement() {
     _treeElements = null;
   }
 }