Fix checeked mode errors in previous commits

tests/standalone/io/http_parser_test.dart

Index: tests/standalone/io/http_parser_test.dart
diff --git a/tests/standalone/io/http_parser_test.dart b/tests/standalone/io/http_parser_test.dart
index 2a6ad1369def075cd411e25d48f8da2ca4ff7010..72f4d8abc2ee494aa29e9bfbdb0c686cc1956785 100644
--- a/tests/standalone/io/http_parser_test.dart
+++ b/tests/standalone/io/http_parser_test.dart
@@ -77,7 +77,7 @@ class HttpParserTest {
       dataEndCalled = false;
       method = null;
       uri = null;
-      headers = new Map();
+      headers = null;
       bytesReceived = 0;
     }
 
@@ -222,7 +222,7 @@ class HttpParserTest {
       dataEndClose = null;
       statusCode = -1;
       reasonPhrase = null;
-      headers = new Map();
+      headers = null;
       bytesReceived = 0;
     }