Make IOSink implement StringSink

tests/standalone/io/http_head_test.dart

Index: tests/standalone/io/http_head_test.dart
diff --git a/tests/standalone/io/http_head_test.dart b/tests/standalone/io/http_head_test.dart
index 60e7321cda3c3afe20c36f5c71e2dd63c460a598..bb33ad30eb3c6a8df0bdf36ada229d2036b288f7 100644
--- a/tests/standalone/io/http_head_test.dart
+++ b/tests/standalone/io/http_head_test.dart
@@ -14,15 +14,15 @@ void testHEAD(int totalConnections) {
         } else if (request.uri.path == "/test200") {
           response.contentLength = 200;
           List<int> data = new List<int>.filled(200, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else if (request.uri.path == "/testChunked100") {
           List<int> data = new List<int>.filled(100, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else if (request.uri.path == "/testChunked200") {
           List<int> data = new List<int>.filled(200, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else {
           assert(false);