Make keySystem argument optional in MediaElement.canPlayType.

tools/dom/idl/dart/dart.idl

 // This file introduces / supplements and forces Dart declarations.
 
 [Supplemental,
  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 166 matching lines @@
     [CallWith=ScriptExecutionContext, RaisesException] IDBRequest openCursor(any key, [ForceOptional] optional DOMString direction);
     [CallWith=ScriptExecutionContext, RaisesException] IDBRequest openKeyCursor([ForceOptional] optional IDBKeyRange? range, [ForceOptional] optional DOMString  direction);
     [CallWith=ScriptExecutionContext, RaisesException] IDBRequest openKeyCursor(any key,
     [ForceOptional] optional DOMString direction);
 
     [CallWith=ScriptExecutionContext, RaisesException] IDBRequest count([ForceOptional] optional IDBKeyRange? range);
     [CallWith=ScriptExecutionContext, RaisesException] IDBRequest count(any key);
 };
 
 [Supplemental]
+interface HTMLMediaElement {
+  DOMString canPlayType([Default=Undefined] optional DOMString type, [Default=Undefined, TreatNullAs=NullString, TreatUndefinedAs=NullString, ForceOptional] optional DOMString keySystem);
+};
+
+[Supplemental]
 interface IDBKeyRange {
   [DartName=only_] static IDBKeyRange only(any value);
   [DartName=lowerBound_] static IDBKeyRange lowerBound(any bound, [ForceOptional] optional boolean open);
   [DartName=upperBound_] static IDBKeyRange upperBound(any bound, [ForceOptional] optional boolean open);
   [DartName=bound_] static IDBKeyRange bound(any lower, any upper, [ForceOptional] optional boolean lowerOpen, [ForceOptional] optional boolean upperOpen);
 };
 [Supplemental]
 interface IDBObjectStore {
     [CallWith=ScriptState, RaisesException] IDBRequest put(any value, [ForceOptional] optional any key);
     [CallWith=ScriptState, RaisesException] IDBRequest add(any value, [ForceOptional] optional any key);
@@ skipping 115 matching lines @@
   [Custom] void reload();
 
   // URI decomposition attributes
   [CustomSetter] attribute DOMString protocol;
   [CustomSetter] attribute DOMString host;
   [CustomSetter] attribute DOMString hostname;
   [CustomSetter] attribute DOMString port;
   [CustomSetter] attribute DOMString pathname;
   [CustomSetter] attribute DOMString search;
   [CustomSetter] attribute DOMString hash;
-};
+};