Add validation to construction of Cookie objects.

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..6d2b6b96db03954add4061160987da15bb27c5c5 100644
--- a/tests/standalone/io/http_headers_test.dart
+++ b/tests/standalone/io/http_headers_test.dart
@@ -436,6 +436,9 @@ void testInvalidCookie() {
   Expect.throws(() => new _Cookie.fromSetCookieValue("xxx"));
   Expect.throws(() => new _Cookie.fromSetCookieValue(
       "xxx=yyy; expires=12 jan 2013"));
+  Expect.throws(() => new _Cookie.fromSetCookieValue("x x = y y"));
+  Expect.throws(() => new _Cookie("[4", "y"));
+  Expect.throws(() => new _Cookie("4", "y\""));
 
   _HttpHeaders headers = new _HttpHeaders("1.1");
   headers.set('Cookie',