Update path-provider plug-in

Address documentation issues, and update to a more appropriate
exception related to the external storage directory functionality.

Author: natelust

packages/path-provider/CHANGELOG.md

@@ -1,6 +1,6 @@
 ## [0.2.1] - 2017-05-11
 
-* Add function to determine external storage directory
+* Add function to determine external storage directory.
 
 ## [0.2.0] - 2017-05-10
 

packages/path-provider/lib/path_provider.dart

@@ -45,13 +45,13 @@ const _channel = const MethodChannel('plugins.flutter.io/path_provider');
   /// The current operating system should be determined before issuing this
   /// function call, as this functionality is only available on Android.
   ///
-  /// On iOS, this function throws an UnimplementedError as it is not possible
+  /// On iOS, this function throws an UnsupportedError as it is not possible
   /// to access outside the app's sandbox.
   ///
   /// On Android this returns getExternalStorageDirectory.
   Future<Directory> getExternalStorageDirectory() async {
-    if (Platform.operatingSystem == "ios")
-      throw new UnimplementedError("Functionality not available on iOS");
+    if (Platform.isIOS)
+      throw new UnsupportedError("Functionality not available on iOS");
     final String path = await _channel.invokeMethod('getStorageDirectory');
     if (path == null)
       return null;