Revert "Revert "Fix x.runtimeType for native classes""

sdk/lib/_internal/compiler/implementation/resolution/members.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 resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Set<Node> get superUses;
 
@@ skipping 3189 matching lines @@
         Link<Node> link = superMixin.mixins.nodes;
         while (!link.isEmpty) {
           supertype = applyMixin(supertype, resolveType(link.head));
           link = link.tail;
         }
         element.supertype = supertype;
       } else {
         element.supertype = resolveSupertype(element, node.superclass);
       }
     }
-
-    // If the super type isn't specified, we make it Object.
-    final objectElement = compiler.objectClass;
-    if (!identical(element, objectElement) && element.supertype == null) {
-      if (objectElement == null) {
-        compiler.internalError("Internal error: cannot resolve Object",
-                               node: node);
-      } else {
-        objectElement.ensureResolved(compiler);
+    // If the super type isn't specified, we provide a default.  The language
+    // specifies [Object] but the backend can pick a specific 'implementation'
+    // of Object - the JavaScript backend chooses between Object and
+    // Interceptor.
+    if (element.supertype == null) {
+      ClassElement superElement = compiler.backend.defaultSuperclass(element);
+      // Avoid making the superclass (usually Object) extend itself.
+      if (element != superElement) {
+        if (superElement == null) {
+          compiler.internalError(
+              "Cannot resolve default superclass for $element",
+              node: node);
+        } else {
+          superElement.ensureResolved(compiler);
+        }
+        element.supertype = superElement.computeType(compiler);
       }
-      element.supertype = objectElement.computeType(compiler);
     }
 
     assert(element.interfaces == null);
     element.interfaces = resolveInterfaces(node.interfaces, node.superclass);
     calculateAllSupertypes(element);
 
     if (node.defaultClause != null) {
       element.defaultClass = visit(node.defaultClause);
     }
     element.addDefaultConstructorIfNeeded(compiler);
@@ skipping 749 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }