Diagnose unbalanced parentheses better.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 const bool VERBOSE = false;
 
 /**
  * A parser event listener that does nothing except throw exceptions
  * on parser errors.
  */
@@ skipping 920 matching lines @@
   }
 
   Token expectedExpression(Token token) {
     listener.cancel("expected an expression, but got '${token.slowToString()}'",
                     token: token);
     pushNode(null);
     return skipToEof(token);
   }
 
   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);
   }
 
   Token expectedBlockToSkip(Token token) {
     if (token.stringValue === 'native') {
       return native.handleNativeBlockToSkip(this, token);
     } else {
       return unexpected(token);
     }
   }
@@ skipping 914 matching lines @@
 
 Node parse(DiagnosticListener diagnosticListener,
            CompilationUnitElement element,
            doParse(Parser parser)) {
   NodeListener listener = new NodeListener(diagnosticListener, element);
   doParse(new Parser(listener));
   Node node = listener.popNode();
   assert(listener.nodes.isEmpty());
   return node;
 }