[Image Picker] Import foundation for the @required annotations (flutter#445)

Author: roughike

packages/image_picker/lib/image_picker.dart

@@ -5,8 +5,8 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
-import 'package:meta/meta.dart';
 
 /// Specifies the source where the picked image should come from.
 enum ImageSource {