Include stack in error about unhandled event.

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 0e03ae66ec42dd0d9141cf5eae139191bcf978ce..ba84564bccfc60afad28f77fff441728f7373544 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -113,8 +113,8 @@ abstract class StackListener extends Listener {
 
   @override
   void logEvent(String name) {
-    print("  ${stack.join('\n  ')}");
-    internalError("Unhandled event: $name in $runtimeType $uri.");
+    internalError("Unhandled event: $name in $runtimeType $uri:\n"
+        "  ${stack.join('\n  ')}");
   }
 
   @override