Diagnose unbalanced parentheses better.

dart/sdk/lib/_internal/compiler/implementation/scanner/string_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;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
@@ skipping 16 matching lines @@
 
   SourceString utf8String(int start, int offset) {
     return new SubstringWrapper(string, start, byteOffset + offset + 1);
   }
 
   void appendByteStringToken(PrecedenceInfo info, SourceString value) {
     // assert(kind != $a || keywords.get(value) == null);
     tail.next = new StringToken.fromSource(info, value, tokenStart);
     tail = tail.next;
   }
+
+  void unmatchedBeginGroup(BeginGroupToken begin) {
+    SourceString error = new SourceString('unmatched "${begin.stringValue}"');
+    Token close =
+        new StringToken.fromSource(BAD_INPUT_INFO, error, begin.charOffset);
+    // We want to ensure that unmatched BeginGroupTokens are reported
+    // as errors. However, the rest of the parser assume the groups
+    // are well-balanced and will never look at the endGroup
+    // token. This is a nice property that allows us to skip quickly
+    // over correct code. By inserting an additional error token in
+    // the stream, we can keep ignoring endGroup tokens.
+    Token next =
+        new StringToken.fromSource(BAD_INPUT_INFO, error, begin.charOffset);
+    begin.endGroup = close;
+    close.next = next;
+    next.next = begin.next;
+  }
 }
 
 class SubstringWrapper implements SourceString {
   final String internalString;
   final int begin;
   final int end;
   int cashedHash = 0;
   String cachedSubString;
 
   SubstringWrapper(String this.internalString,
@@ skipping 33 matching lines @@
     assert(0 <= terminal);
     assert(initial + terminal <= internalString.length);
     return new SubstringWrapper(internalString,
                                 begin + initial, end - terminal);
   }
 
   bool get isEmpty => begin == end;
 
   bool isPrivate() => !isEmpty && identical(internalString.charCodeAt(begin), $_);
 }