Add ClipboardEvent for Firefox.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 2fcf01a5398902463870fe2d85891b19d9508cc7..16fc5cceca784e4952f3c2066052aa6165eb4416 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -11981,7 +11981,7 @@ class ErrorEvent extends Event native "ErrorEvent" {
 
 
 @DomName('Event')
-class Event extends Interceptor native "Event,InputEvent" {
+class Event extends Interceptor native "Event,InputEvent,ClipboardEvent" {
   // In JS, canBubble and cancelable are technically required parameters to
   // init*Event. In practice, though, if they aren't provided they simply
   // default to false (since that's Boolean(undefined)).
@@ -12090,6 +12090,7 @@ class Event extends Interceptor native "Event,InputEvent" {
   @DomName('Event.clipboardData')
   @DocsEditable()
   @SupportedBrowser(SupportedBrowser.CHROME)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @Experimental()
   // Part of copy/paste