Improve error messages.

pkg/front_end/lib/src/fasta/source/stack_listener.dart

Index: pkg/front_end/lib/src/fasta/source/stack_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/stack_listener.dart b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
index 650c0a18285675373865503a31c5dafffcc5c2ea..7b01326d04a04f4a189d5c702b094b887e1cd091 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -252,7 +252,7 @@ abstract class StackListener extends Listener {
   @override
   void handleRecoverableError(Token token, FastaMessage message) {
     debugEvent("Error: ${message.message}");
-    super.handleRecoverableError(token, message);
+    addCompileTimeErrorFromMessage(message);

[ahe, 2017/06/09 08:13:14]

Note: this change also improves recovery from parser errors.

Before this change, these errors would be added to Listener.recoverableErrors
and then checkEmpty would throw an InputError after parsing an element.

Now the error is instead collected by the current library.

   }
 
   void addCompileTimeErrorFromMessage(FastaMessage message);