Support running executables from dependencies.

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';
 import 'utils.dart';
 
 main() {
   initConfig();
-  integration('Errors if the script does not exist.', () {
+  integration('Errors if the script is in an unknown package.', () {
     d.dir(appPath, [
       d.appPubspec()
     ]).create();
 
-    var pub = pubRun(args: ["script"]);
-    pub.stderr.expect("Could not find bin/script.dart.");
-    pub.shouldExit(exit_codes.NO_INPUT);
+    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);
   });
 }