Clean up various ways we find paths in pub.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index a1188805aa05e7857300a17ad0a44f45d639009f..ff10ba3067b7d4e545ad69c67775629703d1a930 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -272,12 +272,8 @@ void _integration(String description, void body(), [Function testFn]) {
 
 /// Get the path to the root "pub/test" directory containing the pub
 /// tests.
-String get testDirectory {
-  var dir = new Options().script;
-  while (path.basename(dir) != 'test') dir = path.dirname(dir);
-
-  return path.absolute(dir);
-}
+String get testDirectory =>
+  path.absolute(path.dirname(libraryPath('test_pub')));
 
 /// Schedules renaming (moving) the directory at [from] to [to], both of which
 /// are assumed to be relative to [sandboxDir].