Diagnose unbalanced parentheses better.

dart/lib/compiler/implementation/scanner/listener.dart

Index: dart/lib/compiler/implementation/scanner/listener.dart
diff --git a/dart/lib/compiler/implementation/scanner/listener.dart b/dart/lib/compiler/implementation/scanner/listener.dart
index 86d088ccf046b8528bc26ee08fb12eecf63370b6..99dd78db6c456972ed6f16c9c52473a8613647ea 100644
--- a/dart/lib/compiler/implementation/scanner/listener.dart
+++ b/dart/lib/compiler/implementation/scanner/listener.dart
@@ -938,7 +938,11 @@ class ElementListener extends Listener {
   }
 
   Token unexpected(Token token) {
-    listener.cancel("unexpected token '${token.slowToString()}'", token: token);
+    String message = "unexpected token '${token.slowToString()}'";
+    if (token.info == BAD_INPUT_INFO) {
+      message = token.stringValue;
+    }
+    listener.cancel(message, token: token);
     return skipToEof(token);
   }