Refactor ConstantSystem

pkg/compiler/lib/src/constants/evaluation.dart

 // Copyright (c) 2015, 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.constants.evaluation;
 
-import '../compiler.dart' show Compiler;
-import '../elements/elements.dart';
+import '../common/backend_api.dart' show BackendClasses;
+import '../core_types.dart' show CommonElements;
 import '../elements/entities.dart';
-import '../elements/resolution_types.dart';
 import '../elements/types.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import 'constructors.dart';
 import 'expressions.dart';
 
 /// Environment used for evaluating constant expressions.
 abstract class Environment {
   // TODO(johnniwinther): Replace this with [CommonElements] and maybe
   // [Backend].
-  Compiler get compiler;
+  CommonElements get commonElements;
+
+  BackendClasses get backendClasses;
 
   /// Read environments string passed in using the '-Dname=value' option.
   String readFromEnvironment(String name);
 
   /// Returns the [ConstantExpression] for the value of the constant [local].
   ConstantExpression getLocalConstant(Local local);
 
   /// Returns the [ConstantExpression] for the value of the constant [field].
   ConstantExpression getFieldConstant(FieldEntity field);
 
@@ skipping 27 matching lines @@
 
   /// Returns the normalized [index]th positional argument.
   ConstantExpression getPositionalArgument(int index) {
     if (index >= callStructure.positionalArgumentCount) {
       // The positional argument is not provided.
       return defaultValues[index];
     }
     return arguments[index];
   }
 }