Unbreak the parser.

dart/lib/compiler/implementation/scanner/parser.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.
 
 /**
  * An event generating parser of Dart programs. This parser expects
  * all tokens in a linked list (aka a token stream).
  *
  * The class [Scanner] is used to generate a token stream. See the
  * file scanner.dart.
@@ skipping 246 matching lines @@
       return true;
     }
     return false;
   }
 
   Token skipBlock(Token token) {
     if (!optional('{', token)) {
       return listener.expectedBlockToSkip(token);
     }
     BeginGroupToken beginGroupToken = token;
-    assert(beginGroupToken.endGroup === null ||
-           beginGroupToken.endGroup.kind === $CLOSE_CURLY_BRACKET);
+    Token endGroup = beginGroupToken.endGroup;
+    if (endGroup === null) {
+      return listener.unmatched(beginGroupToken);
+    } else if (endGroup.kind !== $CLOSE_CURLY_BRACKET) {
+      return listener.unmatched(beginGroupToken);
+    }
     return beginGroupToken.endGroup;
   }
 
   Token parseClass(Token token) {
     Token begin = token;
     listener.beginClassDeclaration(token);
     if (optional('abstract', token)) {
       // TODO(ahe): Notify listener about abstract modifier.
       token = token.next;
     }
@@ skipping 1647 matching lines @@
     }
     listener.handleContinueStatement(hasTarget, continueKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseEmptyStatement(Token token) {
     listener.handleEmptyStatement(token);
     return expectSemicolon(token);
   }
 }