Diagnose unbalanced parentheses better.

dart/sdk/lib/_internal/compiler/implementation/scanner/array_based_scanner.dart

 // Copyright (c) 2011, 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.
 
 part of scanner_implementation;
 
 abstract
 class ArrayBasedScanner<S extends SourceString> extends AbstractScanner<S> {
   int get charOffset => byteOffset + extraCharOffset;
   final Token tokens;
@@ skipping 50 matching lines @@
     tail = tail.next;
   }
 
   void appendEofToken() {
     tail.next = new Token(EOF_INFO, charOffset);
     tail = tail.next;
     // EOF points to itself so there's always infinite look-ahead.
     tail.next = tail;
     discardOpenLt();
     while (!groupingStack.isEmpty) {
-      BeginGroupToken begin = groupingStack.head;
-      begin.endGroup = tail;
+      unmatchedBeginGroup(groupingStack.head);
       groupingStack = groupingStack.tail;
     }
   }
 
   void beginToken() {
     tokenStart = charOffset;
   }
 
   Token firstToken() {
     return tokens.next;
@@ skipping 93 matching lines @@
   void discardOpenLt() {
     while (!groupingStack.isEmpty
         && identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   peek();
 }