Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/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.
 
 part of js_backend;
 
 const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
 class JavaScriptBitNotOperation extends BitNotOperation {
   const JavaScriptBitNotOperation();
@@ skipping 295 matching lines @@
   static const String LENGTH_NAME = "length";
   static const String JS_OBJECT_NAME = "_jsObject";
   static const String KEYS_NAME = "_keys";
   static const String PROTO_VALUE = "_protoValue";
   static const String JS_DATA_NAME = "_jsData";
 
   final ListConstant keyList;
   final Constant protoValue;
   final bool onlyStringKeys;
 
-  JavaScriptMapConstant(DartType type,
+  JavaScriptMapConstant(InterfaceType type,
                         ListConstant keyList,
                         List<Constant> values,
                         this.protoValue,
                         this.onlyStringKeys)
       : this.keyList = keyList,
         super(type, keyList.entries, values);
   bool get isMap => true;
 
   TypeMask computeMask(Compiler compiler) {
     return compiler.typesTask.constMapType;
   }
 
   List<Constant> getDependencies() {
     List<Constant> result = <Constant>[];
     if (onlyStringKeys) {
       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;
   }
 }