Normalize handling of the package root in the runner.

test/runner/browser/loader_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.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/src/backend/state.dart';
@@ skipping 70 matching lines @@
       return liveTest.run().whenComplete(() => liveTest.close());
     });
 
     test("can load and run a failing test", () {
       var liveTest = suite.tests[1].load(suite);
       expectSingleFailure(liveTest);
       return liveTest.run().whenComplete(() => liveTest.close());
     });
   });
 
-  test("throws a nice error if the package root doesn't exist", () {
-    var loader = new Loader([TestPlatform.chrome], root: _sandbox);
-    expect(
-        loader.loadFile(p.join(_sandbox, 'a_test.dart')).first
-            .whenComplete(loader.close),
-        throwsA(isLoadException(
-            "Directory ${p.join(_sandbox, 'packages')} does not exist.")));
-  });
-
   test("loads a suite both in the browser and the VM", () {
     var loader = new Loader([TestPlatform.vm, TestPlatform.chrome],
         root: _sandbox,
         packageRoot: p.join(packageDir, 'packages'));
     var path = p.join(_sandbox, 'a_test.dart');
     return loader.loadFile(path).toList().then((suites) {
       expect(suites[0].platform, equals('VM'));
       expect(suites[0].path, equals(path));
       expect(suites[1].platform, equals('Chrome'));
       expect(suites[1].path, equals(path));
 
       for (var suite in suites) {
         expect(suite.tests, hasLength(3));
         expect(suite.tests[0].name, equals("success"));
         expect(suite.tests[1].name, equals("failure"));
         expect(suite.tests[2].name, equals("error"));
       }
     }).whenComplete(loader.close);
   });
 }