Allow ".dart" extension in pub run.

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('runs a Dart application in the entrypoint package', () {
+  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"]);
+    var pub = pubRun(args: ["script.dart"]);
     pub.stdout.expect("stdout output");
     pub.stderr.expect("stderr output");
     pub.shouldExit(123);
   });
 }