Treat add events for known files as modifications instead of discarding them on Mac OS. Fixes pub s…

lib/src/directory_watcher/mac_os.dart

Index: lib/src/directory_watcher/mac_os.dart
diff --git a/lib/src/directory_watcher/mac_os.dart b/lib/src/directory_watcher/mac_os.dart
index e3efa2dd64bc034aaeed4b1e9369c67171a036c1..ee2013665f57335cff3a7b6e098d996257bb7062 100644
--- a/lib/src/directory_watcher/mac_os.dart
+++ b/lib/src/directory_watcher/mac_os.dart
@@ -179,12 +179,15 @@ class _MacOSDirectoryWatcher implements ManuallyClosedDirectoryWatcher {
       for (var event in events) {
         if (event is FileSystemCreateEvent) {
           if (!event.isDirectory) {
-            // Don't emit ADD events for files or directories that we already
-            // know about. Such an event comes from FSEvents reporting an add
-            // that happened prior to the watch beginning.
-            if (_files.contains(path)) continue;
-
-            _emitEvent(ChangeType.ADD, path);
+            // If we already know about the file, treat it like a modification.

[nweiz, 2015/01/22 00:20:19]

"treat it" -> "treat an ADD event"

+            // This can happen if a file is copied on top of an existing one.
+            // We'll see an ADD event for the latter file when from the user's
+            // perspective, the file's contents just changed.
+            var type = _files.contains(path)
+                ? ChangeType.MODIFY
+                : ChangeType.ADD;
+
+            _emitEvent(type, path);
             _files.add(path);
             continue;
           }