Finish strong mode cleaning of dart2js.

pkg/compiler/lib/src/universe/element_world_builder.dart

Index: pkg/compiler/lib/src/universe/element_world_builder.dart
diff --git a/pkg/compiler/lib/src/universe/element_world_builder.dart b/pkg/compiler/lib/src/universe/element_world_builder.dart
index 6a0ac2653c53cea6d36da9bda5ac556d852de8b3..4b3ae199e8ab54f26b91d6d907206e51c84822fb 100644
--- a/pkg/compiler/lib/src/universe/element_world_builder.dart
+++ b/pkg/compiler/lib/src/universe/element_world_builder.dart
@@ -63,8 +63,8 @@ class ElementResolutionWorldBuilder extends ResolutionWorldBuilderBase {
 
     _instantiationInfo.forEach((cls, info) {
       if (info.instantiationMap != null) {
-        info.instantiationMap
-            .forEach((ConstructorElement constructor, Set<Instance> set) {
+        info.instantiationMap.forEach((_constructor, Set<Instance> set) {
+          ConstructorElement constructor = _constructor;
           for (Instance instance in set) {
             if (instance.isRedirection) {
               continue;