Start pulling pub from its own repo.

sdk/lib/_internal/pub/test/run/allows_dart_extension_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 '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-const SCRIPT = """
-import 'dart:io';
-
-main() {
-  stdout.writeln("stdout output");
-  stderr.writeln("stderr output");
-  exitCode = 123;
-}
-""";
-
-main() {
-  initConfig();
-  integration('allows a ".dart" extension on the argument', () {
-    d.dir(appPath, [
-      d.appPubspec(),
-      d.dir("bin", [
-        d.file("script.dart", SCRIPT)
-      ])
-    ]).create();
-
-    var pub = pubRun(args: ["script.dart"]);
-    pub.stdout.expect("stdout output");
-    pub.stderr.expect("stderr output");
-    pub.shouldExit(123);
-  });
-}