Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/partial_parser_test.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.
 
 import "package:expect/expect.dart";
-import 'options_helper.dart';
 import 'parser_helper.dart';
 
 void main() {
   testSkipExpression();
 }
 
 void testSkipExpression() {
   PartialParser parser = new PartialParser(new Listener());
   Token token = scan('a < b;');
   token = parser.skipExpression(token);
   Expect.equals(';', token.lexeme);
 
   token = scan('[a < b]').next;
   token = parser.skipExpression(token);
   Expect.equals(']', token.lexeme);
 
   token = scan('a < b,');
   token = parser.skipExpression(token);
   Expect.equals(',', token.lexeme);
 }