Cleanup and fixes to some dart:io files

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 d406af1aede3b540a7f566a294ae551f7c8bd686..2219d0f87bee24db924259ba853c27a74c4b15a5 100644
--- a/tests/standalone/io/http_headers_test.dart
+++ b/tests/standalone/io/http_headers_test.dart
@@ -388,6 +388,13 @@ void testInvalidCookie() {
   Expect.throws(() => new _Cookie.fromSetCookieValue("xxx=yyy; expires=12 jan 2013"));
 }
 
+void testHeaderLists() {
+  HttpHeaders.GENERAL_HEADERS.forEach((x) => null);
+  HttpHeaders.ENTITY_HEADERS.forEach((x) => null);
+  HttpHeaders.RESPONSE_HEADERS.forEach((x) => null);
+  HttpHeaders.REQUEST_HEADERS.forEach((x) => null);
+}
+
 main() {
   testMultiValue();
   testDate();
@@ -400,4 +407,5 @@ main() {
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
+  testHeaderLists();
 }