Finish strong mode cleaning of dart2js.

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

 // Copyright (c) 2012, 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.
 
 library dart2js.constant_system.js;
 
 import '../constant_system_dart.dart';
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../common_elements.dart' show CommonElements;
@@ skipping 341 matching lines @@
   }
 
   MapConstantValue createMap(
       CommonElements commonElements,
       InterfaceType sourceType,
       List<ConstantValue> keys,
       List<ConstantValue> values) {
     bool onlyStringKeys = true;
     ConstantValue protoValue = null;
     for (int i = 0; i < keys.length; i++) {
-      var key = keys[i];
+      dynamic key = keys[i];
       if (key.isString) {
         if (key.primitiveValue == JavaScriptMapConstant.PROTO_PROPERTY) {
           protoValue = values[i];
         }
       } else {
         onlyStringKeys = false;
         // Don't handle __proto__ values specially in the general map case.
         protoValue = null;
         break;
       }
@@ skipping 13 matching lines @@
         type, keysList, values, protoValue, onlyStringKeys);
   }
 
   @override
   ConstantValue createSymbol(CommonElements commonElements, String text) {
     InterfaceType type = commonElements.symbolImplementationType;
     FieldEntity field = commonElements.symbolField;
     ConstantValue argument = createString(text);
     // TODO(johnniwinther): Use type arguments when all uses no longer expect
     // a [FieldElement].
-    Map<FieldEntity, ConstantValue> fields = /*<FieldElement, ConstantValue>*/ {
+    Map<FieldEntity, ConstantValue> fields = <dynamic, ConstantValue>{
       field: argument
     };
     return new ConstructedConstantValue(type, fields);
   }
 }
 
 class JavaScriptMapConstant extends MapConstantValue {
   /**
    * The [PROTO_PROPERTY] must not be used as normal property in any JavaScript
    * object. It would change the prototype chain.
@@ skipping 27 matching lines @@
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
 }