Unbreak the parser.

dart/lib/compiler/implementation/scanner/partial_parser.dart

Index: dart/lib/compiler/implementation/scanner/partial_parser.dart
diff --git a/dart/lib/compiler/implementation/scanner/partial_parser.dart b/dart/lib/compiler/implementation/scanner/partial_parser.dart
index 65bc5614400e89ff4e1434bf2d935a415ea17774..bbdaca8421f8a71d7210d7f5e452989bce731c94 100644
--- a/dart/lib/compiler/implementation/scanner/partial_parser.dart
+++ b/dart/lib/compiler/implementation/scanner/partial_parser.dart
@@ -80,9 +80,13 @@ class PartialParser extends Parser {
       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) {