Refactor ConstantSystem

pkg/compiler/lib/src/js_backend/backend.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 js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 3157 matching lines @@
 
   @override
   bool isNativeClass(ClassElement element) {
     return _nativeData.isNative(element);
   }
 
   @override
   bool isNativeMember(MemberElement element) {
     return _nativeData.isNative(element);
   }
+
+  InterfaceType getConstantMapTypeFor(InterfaceType sourceType,
+      {bool hasProtoKey: false, bool onlyStringKeys: false}) {
+    ClassElement classElement = onlyStringKeys
+        ? (hasProtoKey
+            ? helpers.constantProtoMapClass
+            : helpers.constantStringMapClass)
+        : helpers.generalConstantMapClass;
+    List<DartType> typeArgument = sourceType.typeArguments;
+    if (sourceType.treatAsRaw) {
+      return _env.getRawType(classElement);
+    } else {
+      return _env.createInterfaceType(classElement, typeArgument);
+    }
+  }
+
+  @override
+  FieldEntity get symbolField => helpers.symbolImplementationField;
+
+  @override
+  InterfaceType get symbolType {
+    return _env.getRawType(helpers.symbolImplementationClass);
+  }
 }