dart2js: Introduce HGetLength to replace strange use of HFieldGet

pkg/compiler/lib/src/ssa/builder.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
 import '../common.dart';
@@ skipping 5464 matching lines @@
     Element loopVariable = elements.getForInVariable(node);
     SyntheticLocal indexVariable = new SyntheticLocal('_i', loopVariable);
     TypeMask boolType = commonMasks.boolType;
 
     // These variables are shared by initializer, condition, body and update.
     HInstruction array; // Set in buildInitializer.
     bool isFixed; // Set in buildInitializer.
     HInstruction originalLength = null; // Set for growable lists.
 
     HInstruction buildGetLength() {
-      MemberElement lengthElement = helpers.jsIndexableLength;
-      HFieldGet result = new HFieldGet(
-          lengthElement, array, commonMasks.positiveIntType,
+      HInstruction result = new HGetLength(array, commonMasks.positiveIntType,
           isAssignable: !isFixed);
       add(result);
       return result;
     }
 
     void buildConcurrentModificationErrorCheck() {
       if (originalLength == null) return;
       // The static call checkConcurrentModificationError() is expanded in
       // codegen to:
       //
@@ skipping 1224 matching lines @@
       this.oldReturnLocal,
       this.oldReturnType,
       this.oldResolvedAst,
       this.oldStack,
       this.oldLocalsHandler,
       this.inTryStatement,
       this.allFunctionsCalledOnce,
       this.oldElementInferenceResults)
       : super(function);
 }