Make IOSink implement StringSink

tests/standalone/io/http_basic_test.dart

Index: tests/standalone/io/http_basic_test.dart
diff --git a/tests/standalone/io/http_basic_test.dart b/tests/standalone/io/http_basic_test.dart
index eff6f5d056d168e2ad0b4458a017dddb5c57e85e..343852e1b53891971a8003d735271a17e30f237a 100644
--- a/tests/standalone/io/http_basic_test.dart
+++ b/tests/standalone/io/http_basic_test.dart
@@ -110,7 +110,7 @@ class TestServer {
     var response = request.response;
     Expect.equals("GET", request.method);
     request.listen((_) {}, onDone: () {
-        response.addString("01234567890");
+        response.write("01234567890");
         response.close();
       });
   }
@@ -120,7 +120,7 @@ class TestServer {
     var response = request.response;
     response.statusCode = HttpStatus.NOT_FOUND;
     response.headers.set("Content-Type", "text/html; charset=UTF-8");
-    response.addString("Page not found");
+    response.write("Page not found");
     response.close();
   }
 
@@ -228,11 +228,11 @@ void testPOST(bool chunkedEncoding) {
       httpClient.post("127.0.0.1", port, "/echo")
           .then((request) {
             if (chunkedEncoding) {
-              request.addString(data.substring(0, 10));
-              request.addString(data.substring(10, data.length));
+              request.write(data.substring(0, 10));
+              request.write(data.substring(10, data.length));
             } else {
               request.contentLength = data.length;
-              request.addString(data);
+              request.write(data);
             }
             return request.close();
           })