Remove BackendClasses and JavaScriptBackendClasses.

tests/compiler/dart2js/constant_expression_evaluate_test.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.expressions.evaluate_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/common/backend_api.dart';
@@ skipping 53 matching lines @@
     return field.constant;
   }
 
   @override
   ConstantExpression getLocalConstant(LocalVariableElement local) {
     return local.constant;
   }
 
   @override
   CommonElements get commonElements => _compiler.commonElements;
-
-  @override
-  BackendClasses get backendClasses => _compiler.backend.backendClasses;
 }
 
 const List<TestData> DATA = const [
   const TestData('', const [
     const ConstantData('null', const {const {}: 'NullConstant'}),
     const ConstantData('false', const {const {}: 'BoolConstant(false)'}),
     const ConstantData('true', const {const {}: 'BoolConstant(true)'}),
     const ConstantData('0', const {const {}: 'IntConstant(0)'}),
     const ConstantData('0.0', const {const {}: 'DoubleConstant(0.0)'}),
     const ConstantData('"foo"', const {const {}: 'StringConstant("foo")'}),
@@ skipping 202 matching lines @@
           constant.evaluate(environment, DART_CONSTANT_SYSTEM);
       String valueText = value.toStructuredText();
       Expect.equals(
           expectedText,
           valueText,
           "Unexpected value '${valueText}' for contant "
           "`${constant.toDartText()}`, expected '${expectedText}'.");
     });
   });
 }