Wrap Directory.watch on Mac OS for the watcher package.

pkg/watcher/lib/src/utils.dart

Index: pkg/watcher/lib/src/utils.dart
diff --git a/pkg/watcher/lib/src/utils.dart b/pkg/watcher/lib/src/utils.dart
index e4e4457944dc6296890bb202ede4726f55c66fdd..66c0f09cf7b2ea121f9dad780b8a7321d947a076 100644
--- a/pkg/watcher/lib/src/utils.dart
+++ b/pkg/watcher/lib/src/utils.dart
@@ -18,6 +18,10 @@ bool isDirectoryNotFoundException(error) {
   return error.osError.errorCode == notFoundCode;
 }
 
+/// Returns the union of all elements in each set in [sets].
+Set unionAll(Iterable<Set> sets) =>
+  sets.fold(new Set(), (union, set) => union.union(set));
+
 /// Returns a buffered stream that will emit the same values as the stream
 /// returned by [future] once [future] completes.
 ///