Reapply Issue 10911006.

lib/compiler/implementation/js_backend/backend.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.
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
   List<Element> compiledFunctions;
 
@@ skipping 383 matching lines @@
 
   JavaScriptItemCompilationContext() : types = new HTypeMap();
 }
 
 class JavaScriptBackend extends Backend {
   SsaBuilderTask builder;
   SsaOptimizerTask optimizer;
   SsaCodeGeneratorTask generator;
   CodeEmitterTask emitter;
 
+  /**
+   * Interface used to determine if an object has the JavaScript
+   * indexing behavior. The interface is only visible to specific
+   * libraries.
+   */
+  ClassElement jsIndexingBehaviorInterface;
+
   final Map<Element, Map<Element, HType>> fieldInitializers;
   final Map<Element, Map<Element, HType>> fieldConstructorSetters;
   final Map<Element, Map<Element, HType>> fieldSettersType;
 
   final Map<Element, ReturnInfo> returnInfo;
 
   final List<Element> invalidateAfterCodegen;
   ArgumentTypesRegistry argumentTypes;
 
   List<CompilerTask> get tasks {
@@ skipping 18 matching lines @@
     return compiler.findHelper(const SourceString("throwCyclicInit"));
   }
 
   JavaScriptItemCompilationContext createItemCompilationContext() {
     return new JavaScriptItemCompilationContext();
   }
 
   void enqueueHelpers(Enqueuer world) {
     enqueueAllTopLevelFunctions(compiler.jsHelperLibrary, world);
     enqueueAllTopLevelFunctions(compiler.interceptorsLibrary, world);
+
+    jsIndexingBehaviorInterface =
+        compiler.findHelper(const SourceString('JavaScriptIndexingBehavior'));
+    if (jsIndexingBehaviorInterface != null) {
+      world.registerIsCheck(jsIndexingBehaviorInterface.computeType(compiler));
+    }
+
     for (var helper in [const SourceString('Closure'),
                         const SourceString('ConstantMap'),
                         const SourceString('ConstantProtoMap')]) {
       var e = compiler.findHelper(helper);
       if (e !== null) world.registerInstantiatedClass(e);
     }
   }
 
   void codegen(WorkItem work) {
     if (work.element.kind.category == ElementCategory.VARIABLE) {
@@ skipping 249 matching lines @@
           return const SourceString('listSuperTypeCheck');
         }
       } else if (nativeCheck) {
         return const SourceString('callTypeCheck');
       } else {
         return const SourceString('propertyTypeCheck');
       }
     }
   }
 }