Handle watching a non-existent directory.

pkg/watcher/test/directory_watcher_test.dart

Index: pkg/watcher/test/directory_watcher_test.dart
diff --git a/pkg/watcher/test/directory_watcher_test.dart b/pkg/watcher/test/directory_watcher_test.dart
index 9070179a9df6c9407ca852c9d6648415a30b6c93..841dd0808ecc8ccd05a8ed0cfe96b59d62d93691 100644
--- a/pkg/watcher/test/directory_watcher_test.dart
+++ b/pkg/watcher/test/directory_watcher_test.dart
@@ -86,4 +86,23 @@ main() {
     writeFile("a/b/c/d/file.txt");
     expectAddEvent("a/b/c/d/file.txt");
   });
+
+  test('watches a directory created after the watcher', () {
+    // Watch a subdirectory that doesn't exist yet.
+    createWatcher(dir: "a");
+
+    // This implicity creates it.
+    writeFile("a/b/c/d/file.txt");
+    expectAddEvent("a/b/c/d/file.txt");
+  });
+
+  test('when the watched directory is deleted, removes all files', () {
+    writeFile("dir/a.txt");
+    writeFile("dir/b.txt");
+
+    createWatcher(dir: "dir");
+
+    deleteDir("dir");
+    expectRemoveEvents(["dir/a.txt", "dir/b.txt"]);
+  });
 }