Reduce use of Element in optimize.dart

pkg/compiler/lib/src/js_backend/backend_helpers.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.js_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Uris;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
@@ skipping 95 matching lines @@
   ClassElement jsUnmodifiableArrayClass;
   ClassElement jsPositiveIntClass;
   ClassElement jsUInt32Class;
   ClassElement jsUInt31Class;
 
   MemberElement jsIndexableLength;
   Element jsArrayTypedConstructor;
   MethodElement jsArrayRemoveLast;
   MethodElement jsArrayAdd;
   MethodElement jsStringSplit;
-  Element jsStringToString;
-  Element jsStringOperatorAdd;
-  Element objectEquals;
+  MethodElement jsStringToString;
+  MethodElement jsStringOperatorAdd;
+  MethodElement objectEquals;
 
   ClassElement typeLiteralClass;
   ClassElement mapLiteralClass;
   ClassElement constMapLiteralClass;
   ClassElement typeVariableClass;
 
   ClassElement noSideEffectsClass;
   ClassElement noThrowsClass;
   ClassElement noInlineClass;
   ClassElement forceInlineClass;
@@ skipping 737 matching lines @@
   }
 
   MethodElement get toStringForNativeObject {
     return _findHelper('toStringForNativeObject');
   }
 
   MethodElement get hashCodeForNativeObject {
     return _findHelper('hashCodeForNativeObject');
   }
 }