Check overrides.

dart/lib/compiler/implementation/elements/elements.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.
 
 #library('elements');
 
 #import('dart:uri');
 
 #import('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
@@ skipping 256 matching lines @@
 
   bool _isNative = false;
   void setNative() { _isNative = true; }
   bool isNative() => _isNative;
 
   FunctionElement asFunctionElement() => null;
 
   Element cloneTo(Element enclosing, DiagnosticListener listener) {
     listener.cancel("Unimplemented cloneTo", element: this);
   }
+
+  Link<Type> get allSupertypesAndSelf() {
+    return allSupertypes.prepend(new InterfaceType(this));
+  }
 }
 
 class ContainerElement extends Element {
   Link<Element> localMembers = const EmptyLink<Element>();
 
   ContainerElement(name, kind, enclosingElement)
     : super(name, kind, enclosingElement);
 
   void addMember(Element element, DiagnosticListener listener) {
     localMembers = localMembers.prepend(element);
@@ skipping 772 matching lines @@
     bool isPrivate = memberName.isPrivate();
     for (ClassElement s = superclass; s != null; s = s.superclass) {
       // Private members from a different library are not visible.
       if (isPrivate && getLibrary() !== s.getLibrary()) continue;
       Element e = s.lookupLocalMember(memberName);
       if (e === null) continue;
       // Static members are not inherited.
       if (e.modifiers.isStatic()) continue;
       return e;
     }
+    if (isInterface()) {
+      return lookupSuperInterfaceMember(memberName, getLibrary());
+    }
+    return null;
+  }
+
+  Element lookupSuperInterfaceMember(SourceString memberName,
+                                     LibraryElement fromLibrary) {
+    bool isPrivate = memberName.isPrivate();
+    for (Type t in interfaces) {
+      Element e = t.element.lookupLocalMember(memberName);
+      if (e === null) continue;
+      // Private members from a different library are not visible.
+      if (isPrivate && fromLibrary !== e.getLibrary()) continue;
+      // Static members are not inherited.
+      if (e.modifiers.isStatic()) continue;
+      return e;
+    }
     return null;
   }
 
   /**
    * Find the first member in the class chain with the given
    * [memberName]. This method is NOT to be used for resolving
    * unqualified sends because it does not implement the scoping
    * rules, where library scope comes before superclass scope.
    */
   Element lookupMember(SourceString memberName) {
@@ skipping 348 matching lines @@
   Node parseNode(compiler) => cachedNode;
 
   String toString() => "${enclosingElement.toString()}.${name.slowToString()}";
 
   TypeVariableElement cloneTo(Element enclosing, DiagnosticListener listener) {
     TypeVariableElement result =
         new TypeVariableElement(name, enclosing, node, type, bound);
     return result;
   }
 }