Improve error-recovery from unmatched braces.

dart/sdk/lib/_internal/compiler/implementation/warnings.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 dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 1748 matching lines @@
 """
 main() => \"\"\"
 """,
 """
 main() => r'''
 """,
 """
 main() => r\"\"\"
 """]);
 
+  static const MessageKind UNMATCHED_TOKEN = const MessageKind(
+      "Can't find '#{end}' to match '#{begin}'.",
+      howToFix: DONT_KNOW_HOW_TO_FIX,
+      examples: const[
+          "main(",
+          "main(){",
+          "main(){]}",
+        ]);
+
   static const MessageKind UNTERMINATED_TOKEN = const MessageKind(
       // This is a fall-back message that shouldn't happen.
       "Incomplete token.");
 
   static const MessageKind EXPONENT_MISSING = const MessageKind(
       "Numbers in exponential notation should always contain an exponent"
       " (an integer number with an optional sign).",
       howToFix: "Make sure there is an exponent, and remove any whitespace "
       "before it.",
       examples: const ["""
@@ skipping 252 matching lines @@
     return computeMessage();
   }
 
   bool operator==(other) {
     if (other is !Message) return false;
     return (kind == other.kind) && (toString() == other.toString());
   }
 
   int get hashCode => throw new UnsupportedError('Message.hashCode');
 }