Start pulling pub from its own repo.

sdk/lib/_internal/pub/test/run/nonexistent_dependency_test.dart

-// Copyright (c) 2014, 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 '../../lib/src/exit_codes.dart' as exit_codes;
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration('Errors if the script is in an unknown package.', () {
-    d.dir(appPath, [
-      d.appPubspec()
-    ]).create();
-
-    var pub = pubRun(args: ["foo:script"]);
-    pub.stderr.expect('Could not find package "foo". Did you forget to add a '
-        'dependency?');
-    pub.shouldExit(exit_codes.DATA);
-  });
-}