anchor.diff:        var handler = packagesDirHandler();
         var request = new Request(
             "GET",
  -          Uri.parse(
  -              "http://example.com/packages/shelf_packages_handler/"
  +          Uri.parse("http://example.com/packages/shelf_packages_handler/"
                 "non_existent.dart"));
         var response = await handler(request);
         expect(response.statusCode, equals(404));
  
positive.code: void main() {
    var dir;
    setUp(() {
      dir = Directory.systemTemp
          .createTempSync("shelf_packages_handler_test")
          .path;
      new Directory(dir).createSync();
      new Directory("$dir/foo").createSync();
      new File("$dir/foo/foo.dart")
  
negative.code: 
        test("looks up a real file", () async {
          var handler = packagesHandler(resolver: resolver);
          var request =
              new Request("GET", Uri.parse("http://example.com/foo/foo.dart"));
          var response = await handler(request);
          expect(response.statusCode, equals(200));
          expect(await response.readAsString(), contains("in foo"));
  