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

packages/image_picker/lib/image_picker.dart

diff --git a/packages/image_picker/lib/image_picker.dart b/packages/image_picker/lib/image_picker.dart
index e939121..f2b1d83 100755
--- a/packages/image_picker/lib/image_picker.dart
+++ b/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 {