Add fixed content type values for commonly used content types

tests/standalone/io/http_headers_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart.io;
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:collection";
 import "dart:convert";
@@ skipping 307 matching lines @@
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
   contentType = ContentType.parse(
       "  text/html  ;  charset  =  utf-8  ;  xxx=yyy  ");
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
   contentType = ContentType.parse(
       'text/html; charset=utf-8; xxx="yyy"');
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
   contentType = ContentType.parse(
       "  text/html  ;  charset  =  utf-8  ;  xxx=yyy  ");
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
+
+  // Test builtin content types.
+  check(ContentType.TEXT, "text", "plain", {"charset": "utf-8"});
+  check(ContentType.HTML, "text", "html", {"charset": "utf-8"});
+  check(ContentType.JSON, "application", "json", {"charset": "utf-8"});
+  check(ContentType.BINARY, "application", "octet-stream");
 }
 
 void testContentTypeCache() {
   _HttpHeaders headers = new _HttpHeaders("1.1");
   headers.set(HttpHeaders.CONTENT_TYPE, "text/html");
   Expect.equals("text", headers.contentType.primaryType);
   Expect.equals("html", headers.contentType.subType);
   Expect.equals("text/html", headers.contentType.value);
   headers.set(HttpHeaders.CONTENT_TYPE, "text/plain; charset=utf-8");
   Expect.equals("text", headers.contentType.primaryType);
@@ skipping 121 matching lines @@
   testIfModifiedSince();
   testHost();
   testEnumeration();
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
 }