Screenshot API for flutter driver (#6175)

packages/flutter_driver/lib/src/driver.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import 'package:json_rpc_2/json_rpc_2.dart' as rpc;
 import 'package:vm_service_client/vm_service_client.dart';
@@ -267,6 +268,12 @@ class FlutterDriver {
     return GetTextResult.fromJson(await _sendCommand(new GetText(finder))).text;
   }
 
+  /// Take a screenshot.  The image will be returned as a PNG.
+  Future<List<int>> screenshot() async {
+    Map<String, dynamic> result = await _peer.sendRequest('_flutter.screenshot');
+    return BASE64.decode(result['screenshot']);
+  }
+
   /// Starts recording performance traces.
   Future<Null> startTracing({List<TimelineStream> streams: _defaultStreams}) async {
     assert(streams != null && streams.length > 0);