dart2js: Introduce HGetLength to replace strange use of HFieldGet

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

 // Copyright (c) 2016, 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 'package:js_runtime/shared/embedded_names.dart';
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../compiler.dart';
 import '../constants/expressions.dart';
@@ skipping 371 matching lines @@
   ir.Procedure get fallThroughError =>
       kernel.functions[_backend.helpers.fallThroughError];
 
   TypeMask get fallThroughErrorType =>
       TypeMaskFactory.inferredReturnTypeForElement(
           _backend.helpers.fallThroughError, _globalInferenceResults);
 
   ir.Procedure get mapLiteralUntypedMaker =>
       kernel.functions[_backend.helpers.mapLiteralUntypedMaker];
 
-  MemberElement get jsIndexableLength => _backend.helpers.jsIndexableLength;
-
   ir.Procedure get checkConcurrentModificationError =>
       kernel.functions[_backend.helpers.checkConcurrentModificationError];
 
   TypeMask get checkConcurrentModificationErrorReturnType =>
       TypeMaskFactory.inferredReturnTypeForElement(
           _backend.helpers.checkConcurrentModificationError,
           _globalInferenceResults);
 
   ir.Procedure get checkSubtype =>
       kernel.functions[_backend.helpers.checkSubtype];
@@ skipping 482 matching lines @@
       JumpTarget continueTarget =
           astAdapter.getJumpTarget(switchCase, isContinueTarget: true);
       assert(continueTarget is KernelJumpTarget);
       targetIndexMap[continueTarget] = switchIndex;
       assert(builder.jumpTargets[continueTarget] == null);
       builder.jumpTargets[continueTarget] = this;
       switchIndex++;
     }
   }
 }