dart2js: add isConstantsHolder to Holder class.

pkg/compiler/lib/src/js_emitter/program_builder/registry.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/registry.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/registry.dart b/pkg/compiler/lib/src/js_emitter/program_builder/registry.dart
index 2fe3e12e8eafee84b312b5c7ed165a0cc8d869a6..5614bf99c12e512e42e230a15ac32b7fc7c66bd4 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/registry.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/registry.dart
@@ -113,10 +113,17 @@ class Registry {
     // 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);
     });
   }
 }