Fix test.py and tests to use an explicit --packages flag.

tests/standalone/package/scenarios/packages_file_strange_formatting/mixed_line_ends_test.dart

 // Copyright (c) 2015, 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.
 
-// PackageRoot=none
 // Packages=mixed_line_ends.packages
 
 library mixed_line_ends_test;
 
 import 'package:foo/foo.dart' as foo;
 import 'package:bar/bar.dart' as bar;
 import 'package:baz/baz.dart' as baz;
 
 main() {
   if (foo.foo != 'foo') {
     throw new Exception('package "foo" was not resolved correctly');
   }
   if (bar.bar != 'bar') {
     throw new Exception('package "bar" was not resolved correctly');
   }
   if (baz.baz != 'baz') {
     throw new Exception('package "baz" was not resolved correctly');
   }
 }