Don't hide interceptors in mirrors.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 0942a3ab39f713f6de3d14e9c6c1f65693b2c9ec..cb0912fa0f7e8192ab5c78865a32e52fba1b9cc6 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -558,6 +558,8 @@ class JavaScriptBackend extends Backend {
         ..add(jsNullClass);
 
     validateInterceptorImplementsAllObjectMethods(jsInterceptorClass);
+    // The null-interceptor must also implement *all* methods.
+    validateInterceptorImplementsAllObjectMethods(jsNullClass);
 
     typeVariableClass = compiler.findHelper('TypeVariable');
 
@@ -583,7 +585,7 @@ class JavaScriptBackend extends Backend {
       Element interceptorMember = interceptorClass.lookupMember(member.name);
       // Interceptors must override all Object methods due to calling convention
       // differences.
-      assert(interceptorMember.getEnclosingClass() != compiler.objectClass);
+      assert(interceptorMember.getEnclosingClass() == interceptorClass);
     });
   }
 
@@ -1110,7 +1112,8 @@ class JavaScriptBackend extends Backend {
     assert(element.name == Compiler.NO_SUCH_METHOD);
     ClassElement classElement = element.getEnclosingClass();
     return classElement == compiler.objectClass
-        || classElement == jsInterceptorClass;
+        || classElement == jsInterceptorClass
+        || classElement == jsNullClass;
   }
 
   bool isDefaultEqualityImplementation(Element element) {
@@ -1264,15 +1267,6 @@ class JavaScriptBackend extends Backend {
     return element;
   }
 
-  Element getImplementationClass(Element element) {
-    for (ClassElement dartClass in implementationClasses.keys) {
-      if (element == dartClass) {
-        return implementationClasses[dartClass];
-      }
-    }
-    return element;
-  }
-
   /**
    * Returns the checked mode helper that will be needed to do a type check/type
    * cast on [type] at runtime. Note that this method is being called both by