Make file system watcher compile on Mac OS 106, and add a runtime-call to test if the system suppor…

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index 8d86fe8f5d0f06b47fa57023ae309b32ffe06e37..ee088cc7ce7ee6a4dee91827802ac92cbeed2832 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -344,6 +344,13 @@ abstract class FileSystemEntity {
                                events,
                                recursive).stream;
 
+  /**
+   * Test if [watch] is supported on the current system.
+   *
+   * Mac OS 10.6 and below is not supported.
+   */
+  static bool get isWatchSupported => _FileSystemWatcher.isSupported;
+
 
   /**
    * Finds the type of file system object that a path points to. Returns
@@ -537,6 +544,7 @@ class FileSystemMoveEvent extends FileSystemEvent {
 
 abstract class _FileSystemWatcher {
   external factory _FileSystemWatcher(String path, int events, bool recursive);
+  external static bool get isSupported;
 
   Stream<FileSystemEvent> get stream;
 }