dart2js: Default to an empty set instead of null

pkg/compiler/lib/src/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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 922 matching lines @@
   /// Maps compile-time classes to their runtime class.  The runtime class is
   /// always a superclass or the class itself.
   ClassElement getRuntimeClass(ClassElement class_) {
     if (class_.isSubclassOf(helpers.jsIntClass)) return helpers.jsIntClass;
     if (class_.isSubclassOf(helpers.jsArrayClass)) return helpers.jsArrayClass;
     return class_;
   }
 
   final Map<String, Set<ClassElement>> interceptedClassesCache =
       new Map<String, Set<ClassElement>>();
+  final Set<ClassElement> _noClasses = new Set<ClassElement>();
 
-  /**
-   * Returns a set of interceptor classes that contain a member named
-   * [name]. Returns [:null:] if there is no class.
-   */
+  /// Returns a set of interceptor classes that contain a member named [name]
+  ///
+  /// Returns an empty set if there is no class. Do not modify the returned set.
   Set<ClassElement> getInterceptedClassesOn(String name) {
     Set<Element> intercepted = interceptedElements[name];
-    if (intercepted == null) return null;
+    if (intercepted == null) return _noClasses;
     return interceptedClassesCache.putIfAbsent(name, () {
       // Populate the cache by running through all the elements and
       // determine if the given selector applies to them.
       Set<ClassElement> result = new Set<ClassElement>();
       for (Element element in intercepted) {
         ClassElement classElement = element.enclosingClass;
         if (isCompileTimeOnlyClass(classElement)) continue;
         if (isNativeOrExtendsNative(classElement)
             || interceptedClasses.contains(classElement)) {
           result.add(classElement);
@@ skipping 2203 matching lines @@
     }
   }
 
   @override
   void onImpactUsed(ImpactUseCase impactUse) {
     if (impactUse == DeferredLoadTask.IMPACT_USE) {
       resolution.emptyCache();
     }
   }
 }