Make .enclosingClass always return the declaration

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 3975 matching lines @@
       List<HInstruction> arguments,
       {SourceInformation sourceInformation}) {
     // We prefer to not inline certain operations on indexables,
     // because the constant folder will handle them better and turn
     // them into simpler instructions that allow further
     // optimizations.
     bool isOptimizableOperationOnIndexable(Selector selector, Element element) {
       bool isLength = selector.isGetter && selector.name == "length";
       if (isLength || selector.isIndex) {
         return closedWorld.isSubtypeOf(
-            element.enclosingClass.declaration, helpers.jsIndexableClass);
+            element.enclosingClass, helpers.jsIndexableClass);
       } else if (selector.isIndexSet) {
-        return closedWorld.isSubtypeOf(element.enclosingClass.declaration,
-            helpers.jsMutableIndexableClass);
+        return closedWorld.isSubtypeOf(
+            element.enclosingClass, helpers.jsMutableIndexableClass);
       } else {
         return false;
       }
     }
 
     bool isOptimizableOperation(Selector selector, Element element) {
       ClassElement cls = element.enclosingClass;
       if (isOptimizableOperationOnIndexable(selector, element)) return true;
       if (!backend.interceptedClasses.contains(cls)) return false;
       if (selector.isOperator) return true;
@@ skipping 2759 matching lines @@
       this.oldReturnLocal,
       this.oldReturnType,
       this.oldResolvedAst,
       this.oldStack,
       this.oldLocalsHandler,
       this.inTryStatement,
       this.allFunctionsCalledOnce,
       this.oldElementInferenceResults)
       : super(function);
 }