Report external without patch as an error.

sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart

Index: sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart
diff --git a/sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart b/sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart
index f80a616e6d1db177572f3551072f27264752af2b..8f3785db11b9e0059481d72cec5a0bfbc11f3cc4 100644
--- a/sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/diagnostic_listener.dart
@@ -16,7 +16,7 @@ abstract class DiagnosticListener {
                      {Node node, Token token, HInstruction instruction,
                       Element element});
 
-  SourceSpan spanFromSpannable(Node node, [Uri uri]);
+  SourceSpan spanFromSpannable(Spannable node, [Uri uri]);
 
   void reportMessage(SourceSpan span, Diagnostic message, api.Diagnostic kind);