Move constant systems into their own libraries.

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

Index: pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
index deadc5f7549f5390c579fd4e3cc5bb1a1f5913f7..fc97cadc60b7ff121ba9edcd4fbb0b0469f24c42 100644
--- a/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_system_javascript.dart
@@ -2,7 +2,16 @@
 // 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;
+library dart2js.constant_system.js;
+
+import '../constants/constant_system.dart';
+import '../constants/values.dart';
+import '../constant_system_dart.dart';
+import '../dart_types.dart';
+import '../dart2jslib.dart' show Compiler;
+import '../elements/elements.dart' show ClassElement;
+import '../tree/tree.dart' show DartString, LiteralDartString;
+import 'js_backend.dart';
 
 const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
@@ -343,10 +352,6 @@ class JavaScriptMapConstant extends MapConstantValue {
         super(type, keyList.entries, values);
   bool get isMap => true;
 
-  TypeMask computeMask(Compiler compiler) {
-    return compiler.typesTask.constMapType;
-  }
-
   List<ConstantValue> getDependencies() {
     List<ConstantValue> result = <ConstantValue>[];
     if (onlyStringKeys) {