[dart:html] minor fix to clearWatch.

This is a fix to not include a null assert when it's
possible the value was null.

Fixes https://github.com/dart-lang/sdk/issues/49972

CoreLibraryReviewExempt: bug fix only affecting html library implementation
Change-Id: I26e6ebf3dc2d1ea3f9f458a32f519c5235e6d273
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350982
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>

tools/dom/templates/html/impl/impl_Geolocation.darttemplate

@@ -52,8 +52,8 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
     StreamController<Geoposition> controller =
       new StreamController<Geoposition>(sync: true,
         onCancel: () {
-          assert(watchId != null);
-          _clearWatch(watchId$NULLASSERT);
+          final id = watchId;
+          if (id != null) _clearWatch(id);
         }
       );
     controller.onListen = () {