Make instances of HeaderValue and ContentType immutable

tests/standalone/io/http_advanced_test.dart

Index: tests/standalone/io/http_advanced_test.dart
diff --git a/tests/standalone/io/http_advanced_test.dart b/tests/standalone/io/http_advanced_test.dart
index e19365f66a892fb3c4a7e1f6b2152da9357ad310..b62c5d848d9f184028dc4361df0fe81e0665bb01 100644
--- a/tests/standalone/io/http_advanced_test.dart
+++ b/tests/standalone/io/http_advanced_test.dart
@@ -347,10 +347,8 @@ Future testContentType() {
 
     httpClient.get("127.0.0.1", port, "/contenttype1")
         .then((request) {
-          ContentType contentType = new ContentType();
-          contentType.value = "text/html";
-          contentType.parameters["charset"] = "utf-8";
-          request.headers.contentType = contentType;
+          request.headers.contentType =
+              new ContentType("text", "html", "utf-8");
           return request.close();
         })
         .then(processResponse);