Add fixed content type values for commonly used content types

tests/standalone/io/http_headers_test.dart

Index: tests/standalone/io/http_headers_test.dart
diff --git a/tests/standalone/io/http_headers_test.dart b/tests/standalone/io/http_headers_test.dart
index 7417d56382cda6c26a818271f2718d82fa4a391b..a4ea1a3e380832b38371b3a7e1ef4da790cb74b0 100644
--- a/tests/standalone/io/http_headers_test.dart
+++ b/tests/standalone/io/http_headers_test.dart
@@ -325,6 +325,12 @@ void testContentType() {
   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() {