dart2js: add isConstantsHolder to Holder class.

pkg/compiler/lib/src/js_emitter/program_builder/registry.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 dart2js.js_emitter.program_builder;
 
 /// Maps [LibraryElement]s to their [Element]s.
 ///
 /// Fundamentally, this class nicely encapsulates a
 /// `Map<LibraryElement, List<Element>>`.
@@ skipping 95 matching lines @@
     LibrariesMap targetLibrariesMap = _mapUnitToLibrariesMap(outputUnit);
     for (Element element in Elements.sortedByPosition(elements)) {
       targetLibrariesMap.add(element.library, element);
     }
   }
 
   void registerConstant(OutputUnit outputUnit, ConstantValue constantValue) {
     // Ignore for now.
   }
 
-  Holder registerHolder(String name, {bool isStaticStateHolder: false}) {
+  Holder registerHolder(
+      String name,
+      {bool isStaticStateHolder: false, bool isConstantsHolder: false}) {
+    assert(_holdersMap[name] == null ||
+        (_holdersMap[name].isStaticStateHolder == isStaticStateHolder &&
+         _holdersMap[name].isConstantsHolder == isConstantsHolder));
+
     return _holdersMap.putIfAbsent(name, () {
       return new Holder(name, _holdersMap.length,
-          isStaticStateHolder: isStaticStateHolder);
+          isStaticStateHolder: isStaticStateHolder,
+          isConstantsHolder: isConstantsHolder);
     });
   }
 }