Disable assertion and remove type to make host-checked mode green again.

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.
 
 part of scanner;
 
 /**
  * An event generating parser of Dart programs. This parser expects
  * all tokens in a linked list (aka a token stream).
  *
@@ skipping 1522 matching lines @@
     } else {
       bool old = mayParseFunctionExpressions;
       mayParseFunctionExpressions = true;
       token = parseParenthesizedExpression(token);
       mayParseFunctionExpressions = old;
       return token;
     }
   }
 
   Token parseParenthesizedExpression(Token token) {
-    BeginGroupToken begin = token;
+    var begin = token;
     token = expect('(', token);
     token = parseExpression(token);
     if (!identical(begin.endGroup, token)) {
       listener.unexpected(token);
       token = begin.endGroup;
     }
     listener.handleParenthesizedExpression(begin);
     return expect(')', token);
   }
 
@@ skipping 608 matching lines @@
     }
     listener.handleContinueStatement(hasTarget, continueKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseEmptyStatement(Token token) {
     listener.handleEmptyStatement(token);
     return expectSemicolon(token);
   }
 }