Fix a Dart 2 runtime test failure (flutter#14)

Author: nex3

test/byte_accumulator_sink_test.dart

@@ -6,7 +6,7 @@ import 'package:convert/convert.dart';
 import 'package:test/test.dart';
 
 void main() {
-  var sink;
+  ByteAccumulatorSink sink;
   setUp(() {
     sink = new ByteAccumulatorSink();
   });