Implement override checks for methods.

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

Index: pkg/front_end/lib/src/fasta/messages.dart
diff --git a/pkg/front_end/lib/src/fasta/messages.dart b/pkg/front_end/lib/src/fasta/messages.dart
index 5e03fd567f3ccdc53bd2af68e25f6be28535d8d6..d1f31aaa73b14a8d2a64a22aa809b6e806295a0f 100644
--- a/pkg/front_end/lib/src/fasta/messages.dart
+++ b/pkg/front_end/lib/src/fasta/messages.dart
@@ -70,6 +70,9 @@ String format(Uri uri, int charOffset, String 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);
 }