Handle HTTP header parameters with empty values better

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 1c84cd51fefe4eda662b03d3c78e0e0349c83484..26edb5fe1855770eeab7bd9ed6239d2170293e4d 100644
--- a/tests/standalone/io/http_headers_test.dart
+++ b/tests/standalone/io/http_headers_test.dart
@@ -380,6 +380,11 @@ void testContentType() {
       "  text/html  ;  charset  =  utf-8  ;  xxx=yyy  ");
   check(contentType, "text", "html", {"charset": "utf-8", "xxx": "yyy"});
 
+  contentType = ContentType.parse("text/html; charset=;");
+  check(contentType, "text", "html", {"charset": null});
+  contentType = ContentType.parse("text/html; charset;");
+  check(contentType, "text", "html", {"charset": null});
+
   // Test builtin content types.
   check(ContentType.TEXT, "text", "plain", {"charset": "utf-8"});
   check(ContentType.HTML, "text", "html", {"charset": "utf-8"});
@@ -387,6 +392,20 @@ void testContentType() {
   check(ContentType.BINARY, "application", "octet-stream");
 }
 
+void testKnownContentTypes() {
+  // Well known content types used by the VM service.
+  ContentType.parse('text/html; charset=UTF-8');
+  ContentType.parse('application/dart; charset=UTF-8');
+  ContentType.parse('application/javascript; charset=UTF-8');
+  ContentType.parse('text/css; charset=UTF-8');
+  ContentType.parse('image/gif');
+  ContentType.parse('image/png');
+  ContentType.parse('image/jpeg');
+  ContentType.parse('image/jpeg');
+  ContentType.parse('image/svg+xml');
+  ContentType.parse('text/plain');
+}
+
 void testContentTypeCache() {
   _HttpHeaders headers = new _HttpHeaders("1.1");
   headers.set(HttpHeaders.CONTENT_TYPE, "text/html");
@@ -570,6 +589,7 @@ main() {
   testEnumeration();
   testHeaderValue();
   testContentType();
+  testKnownContentTypes();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();