Add a validator that checks that the lib directory is not empty.

utils/tests/pub/test_pub.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Test infrastructure for testing pub. Unlike typical unit tests, most pub
  * tests are integration tests that stage some stuff on the file system, run
  * pub, and then validate the results. This library provides an API to build
  * tests like that.
  */
@@ skipping 204 matching lines @@
           ])
         ]);
       }
     });
   });
 }
 
 /** Converts [value] into a YAML string. */
 String yaml(value) => JSON.stringify(value);
 
+/// Describes a package that passes all validation.
+Descriptor get normalPackage => dir(appPath, [
+  libPubspec("test_pkg", "1.0.0"),
+  file("LICENSE", "Eh, do what you want."),
+  dir("lib", [
+    file("test_pkg.dart", "int i = 1;")
+  ])
+]);
+
 /**
  * Describes a file named `pubspec.yaml` with the given YAML-serialized
  * [contents], which should be a serializable object.
  *
  * [contents] may contain [Future]s that resolve to serializable objects, which
  * may in turn contain [Future]s recursively.
  */
 Descriptor pubspec(Map contents) {
   return async(_awaitObject(contents).transform((resolvedContents) =>
       file("pubspec.yaml", yaml(resolvedContents))));
@@ skipping 1397 matching lines @@
 /// calling [completion] is unnecessary.
 void expectLater(Future actual, matcher, {String reason,
     FailureHandler failureHandler, bool verbose: false}) {
   _schedule((_) {
     return actual.transform((value) {
       expect(value, matcher, reason: reason, failureHandler: failureHandler,
           verbose: false);
     });
   });
 }