Perform override and inheritance checks.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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.modelx;
 
 import 'elements.dart';
 import '../../compiler.dart' as api;
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 588 matching lines @@
     if (enclosingElement.isPatch) {
       getImplementationLibrary().addMember(element, listener);
     } else {
       getLibrary().addMember(element, listener);
     }
   }
 
   void setPartOf(PartOf tag, DiagnosticListener listener) {
     LibraryElementX library = enclosingElement;
     if (library.entryCompilationUnit == this) {
-      listener.reportMessage(
-          listener.spanFromSpannable(tag),
-          MessageKind.ILLEGAL_DIRECTIVE.error(),
-          api.Diagnostic.WARNING);
+      listener.reportError(tag, MessageKind.ILLEGAL_DIRECTIVE);
       return;
     }
     if (!localMembers.isEmpty) {
       listener.reportError(tag, MessageKind.BEFORE_TOP_LEVEL);
       return;
     }
     if (partTag != null) {
       listener.reportMessage(
           listener.spanFromSpannable(tag),
           MessageKind.DUPLICATED_PART_OF.error(),
@@ skipping 1841 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }