Make html metadata follow the spec.

sdk/lib/html/html_common/metadata.dart

Index: sdk/lib/html/html_common/metadata.dart
diff --git a/sdk/lib/html/html_common/metadata.dart b/sdk/lib/html/html_common/metadata.dart
index 82e0be95ab559b06aa92f9538d1aaeccc6a23733..9b0e56da5e68d3c721ee1619ae3bf4890db70bfc 100644
--- a/sdk/lib/html/html_common/metadata.dart
+++ b/sdk/lib/html/html_common/metadata.dart
@@ -38,7 +38,9 @@ class SupportedBrowser {
  *
  * * [W3C recommendation](http://en.wikipedia.org/wiki/W3C_recommendation)
  */
-class Experimental {}
+class Experimental {
+  const Experimental();
+}
 
 
 /**
@@ -58,11 +60,15 @@ class DomName {
  * Metadata that specifies that that member is editable through generated
  * files.
  */
-class DocsEditable {}
+class DocsEditable {
+  const DocsEditable();
+}
 
 
 /**
  * Annotation that indicates that an API is not expected to change but has
  * not undergone enough testing to be considered stable.
  */
-class Unstable {}
+class Unstable {
+  const Unstable();
+}