Revert to using window.onload

To support top level await, jasmine-browser-runner needs to be able
to delay env execution until after spec files have initialized. The
old-fashioned event listener style makes that straightforward.

lib/jasmine-core/boot1.js

@@ -49,11 +49,16 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
    */
   env.configure(urls.configFromCurrentUrl());
 
-  window.addEventListener('load', function() {
+  const currentWindowOnload = window.onload;
+  window.onload = function() {
+    if (currentWindowOnload) {
+      currentWindowOnload();
+    }
+
     // The HTML reporter needs to be set up here so it can access the DOM. Other
     // reporters can be added at any time before env.execute() is called.
     const htmlReporter = new jasmine.HtmlReporterV2({ env, urls });
     env.addReporter(htmlReporter);
     env.execute();
-  });
+  };
 })();