Reduce devtools_test.dart flakiness (#1086)

The `can not launch devtools for the same app in multiple tabs` test appears to be flaky because we attempt to launch DevTools before the page is fully loaded. There isn't a good asynchronous signal for this so as an alternative we wait for the page content to say `Hello World!`. This  closes #1084.

Author: grouma

dwds/test/devtools_test.dart

@@ -14,6 +14,16 @@ final context = TestContext(
   path: 'append_body/index.html',
 );
 
+Future<void> _waitForPageReady(TestContext context) async {
+  var attempt = 100;
+  while (attempt-- > 0) {
+    var content = await context.webDriver.pageSource;
+    if (content.contains('Hello World!')) return;
+    await Future.delayed(const Duration(milliseconds: 100));
+  }
+  throw StateError('Page never initialized');
+}
+
 void main() {
   group('Injected client', () {
     setUp(() async {
@@ -47,6 +57,9 @@ void main() {
       var devToolsWindow = windows[2];
       await newAppWindow.setAsActive();
 
+      // Wait for the page to be ready before trying to open DevTools again.
+      await _waitForPageReady(context);
+
       // Try to open devtools and check for the alert.
       await context.webDriver.driver.keyboard.sendChord([Keyboard.alt, 'd']);
       await Future.delayed(const Duration(seconds: 2));