Begin to compute constant expressions in resolution.

pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart

 // Copyright (c) 2014, 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 js_backend;
 
 /// [ConstantCompilerTask] for compilation of constants for the JavaScript
 /// backend.
 ///
 /// Since this task needs to distinguish between frontend and backend constants
@@ skipping 182 matching lines @@
 
   ConstantExpression compileNode(Node node, TreeElements elements,
                                  {bool enforceConst: true}) {
     return compileNodeWithDefinitions(node, elements, isConst: enforceConst);
   }
 
   ConstantExpression compileNodeWithDefinitions(Node node,
                                       TreeElements definitions,
                                       {bool isConst: true}) {
     ConstantExpression constant = nodeConstantMap[node];
-    if (constant != null) {
+    if (constant != null && getConstantValue(constant) != null) {
       return constant;
     }
     constant =
         super.compileNodeWithDefinitions(node, definitions, isConst: isConst);
     if (constant != null) {
       nodeConstantMap[node] = constant;
     }
     return constant;
   }
 
@@ skipping 66 matching lines @@
     // TODO(ahe): This doesn't belong here. Rename this class and generalize.
     var closureClassMap =
         constants.compiler.closureToClassMapper.closureMappingCache
         .remove(node);
     if (closureClassMap != null) {
       closureClassMap.removeMyselfFrom(
           constants.compiler.enqueuer.codegen.universe);
     }
   }
 }