Unbreak the parser.

dart/lib/compiler/implementation/scanner/partial_parser.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.
 
 class PartialParser extends Parser {
   PartialParser(Listener listener) : super(listener);
 
   Token parseClassBody(Token token) => skipClassBody(token);
 
   Token fullParseClassBody(Token token) => super.parseClassBody(token);
@@ skipping 62 matching lines @@
       }
       token = token.next;
     }
   }
 
   Token skipClassBody(Token token) {
     if (!optional('{', token)) {
       return listener.expectedClassBodyToSkip(token);
     }
     BeginGroupToken beginGroupToken = token;
-    assert(beginGroupToken.endGroup === null ||
-           beginGroupToken.endGroup.kind === $CLOSE_CURLY_BRACKET);
-    return beginGroupToken.endGroup;
+    Token endGroup = beginGroupToken.endGroup;
+    if (endGroup === null) {
+      return listener.unmatched(beginGroupToken);
+    } else if (endGroup.kind !== $CLOSE_CURLY_BRACKET) {
+      return listener.unmatched(beginGroupToken);
+    }
+    return endGroup;
   }
 
   Token parseFunctionBody(Token token, bool isExpression) {
     assert(!isExpression);
     String value = token.stringValue;
     if (value === ';') {
       // No body.
     } else if (value === '=>') {
       token = parseExpression(token.next);
       expectSemicolon(token);
@@ skipping 16 matching lines @@
         return token;
       }
       return listener.unexpected(token);
     }
     BeginGroupToken beginGroupToken = token;
     Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken);
     return endToken.next;
   }
 }