Split DragEvent back out from MouseEvent

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [DartSupplemental,
  Constructor]
 interface AudioContext {
   // TODO(ager): Auto-generate this custom method when the info about retaining
   // typed arrays is in the IDL.
   [Custom] void decodeAudioData(ArrayBuffer audioData, AudioBufferCallback successCallback, AudioBufferCallback errorCallback);
 };
 
@@ skipping 445 matching lines @@
     [DartName=inch] readonly attribute double? in;
 };
 
 [DartSupplemental]
 interface RTCPeerConnection {
     [DartSuppress] Promise<void> setLocalDescription(RTCSessionDescriptionInit description);
     [DartSuppress] Promise<void> setRemoteDescription(RTCSessionDescriptionInit description);
 };
 
 [DartSuppress]
-interface DragEvent {};
-
-[DartSuppress]
 interface InputEvent {};
 
 Element implements GlobalEventHandlers;
 
 
 // This only exists in Safari, and is deprecated there. But include it anyway
 // because otherwise iterating CSSRules in Safari can yield unknown JS objects.
 interface CSSCharsetRule : CSSRule {
     [MeasureAs=CSSCharsetRuleEncoding] attribute DOMString encoding;
 };