Rewrite file-system-watcher to better handle the different system APIs.

runtime/bin/file_system_watcher.h

Index: runtime/bin/file_system_watcher.h
diff --git a/runtime/bin/file_system_watcher.h b/runtime/bin/file_system_watcher.h
index 21c334f771f4b230ef35656e7bea510b93257b86..6a6164a087ba3dd667acbd0608d0e7f86546ac99 100644
--- a/runtime/bin/file_system_watcher.h
+++ b/runtime/bin/file_system_watcher.h
@@ -37,10 +37,15 @@ class FileSystemWatcher {
   };
 
   static bool IsSupported();
-  static intptr_t WatchPath(const char* path, int events, bool recursive);
-  static void UnwatchPath(intptr_t id);
-  static intptr_t GetSocketId(intptr_t id);
-  static Dart_Handle ReadEvents(intptr_t id);
+  static intptr_t Init();
+  static void Close(intptr_t id);
+  static intptr_t WatchPath(intptr_t id,
+                            const char* path,
+                            int events,
+                            bool recursive);
+  static void UnwatchPath(intptr_t id, intptr_t path_id);
+  static intptr_t GetSocketId(intptr_t id, intptr_t path_id);
+  static Dart_Handle ReadEvents(intptr_t id, intptr_t path_id);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FileSystemWatcher);