dart2js: Use an abstract Name class for names in the generated JavaScript ast.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

 // Copyright (c) 2013, 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.ir_builder;
 
 import '../compile_time_constants.dart' show BackendConstantEnvironment;
 import '../constants/constant_system.dart';
-import '../constants/expressions.dart';
 import '../constants/values.dart' show ConstantValue, PrimitiveConstantValue;
 import '../dart_types.dart';
 import '../dart2jslib.dart';
 import '../elements/elements.dart';
 import '../io/source_information.dart';
 import '../tree/tree.dart' as ast;
 import '../closure.dart' hide ClosureScope;
 import '../universe/universe.dart' show SelectorKind;
 import 'cps_ir_nodes.dart' as ir;
 import 'cps_ir_builder_task.dart' show DartCapturedVariables,
@@ skipping 2611 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }