Move more js_util usage to the dom library (#2121)

This was originally written to use `js_util.getProperty` at the calling
side to push the knowledge of the different types of sources into the
usage location. Instead, wrap it up with js interop and use nullability
to reflect that some of the fields may not exist.

pkgs/test/CHANGELOG.md

@@ -1,3 +1,5 @@
+## 1.24.9-wip
+
 ## 1.24.8
 
 * Remove spurious deprecation during autocomplete for `setUp` and `tearDown`.

pkgs/test/lib/src/runner/browser/dom.dart

@@ -161,6 +161,21 @@ extension MessageEventExtension on MessageEvent {
 @staticInterop
 class MessageEventSource {}
 
+extension MessageEventSourceExtension on MessageEventSource {
+  @pragma('dart2js:as:trust')
+  MessageEventSourceLocation? get location =>
+      js_util.getProperty<dynamic>(this, 'location')
+          as MessageEventSourceLocation;
+}
+
+@JS()
+@staticInterop
+class MessageEventSourceLocation {}
+
+extension MessageEventSourceLocationExtension on MessageEventSourceLocation {
+  external String? get href;
+}
+
 @JS()
 @staticInterop
 class Location {}