Implement override checks for methods.

pkg/front_end/lib/src/fasta/messages.dart

 // Copyright (c) 2016, 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 fasta.messages;
 
 import 'package:kernel/ast.dart' show Location, Program;
 
 import 'util/relativize.dart' show relativizeUri;
 
@@ skipping 52 matching lines @@
     String path = relativizeUri(uri);
     String position =
         charOffset == -1 ? path : "${getLocation(path, charOffset)}";
     return "$position: $message";
   } else {
     return message;
   }
 }
 
 Location getLocation(String path, int charOffset) {
+  if (CompilerContext.current.uriToSource[path] == null) {
+    return new Location(path, 1, 1);
+  }
   return new Program(null, CompilerContext.current.uriToSource)
       .getLocation(path, charOffset);
 }