Listen for uncaught exceptions during loading of a web test suite in Chrome (flutter/engine#55166)

Without this the test runner will hang if the compiled test is invalid and unable to execute.

engine/src/flutter/lib/web_ui/dev/browser.dart

@@ -75,6 +75,12 @@ abstract class Browser {
   /// with an error.
   Future<void> get onExit;
 
+  /// A future that completes if the browser is notified about an uncaught
+  /// exception.
+  ///
+  /// Returns `null` if the browser does not support this.
+  Future<String>? get onUncaughtException => null;
+
   /// Closes the browser
   ///
   /// Returns the same [Future] as [onExit], except that it won't emit

engine/src/flutter/lib/web_ui/dev/chrome.dart

@@ -20,6 +20,8 @@ import 'common.dart';
 import 'environment.dart';
 import 'package_lock.dart';
 
+const String kBlankPageUrl = 'about:blank';
+
 /// Provides an environment for desktop Chrome.
 class ChromeEnvironment implements BrowserEnvironment {
   ChromeEnvironment({
@@ -82,6 +84,7 @@ class Chrome extends Browser {
     required bool useDwarf,
   }) {
     final Completer<Uri> remoteDebuggerCompleter = Completer<Uri>.sync();
+    final Completer<String> exceptionCompleter = Completer<String>();
     return Chrome._(BrowserProcess(() async {
       // A good source of various Chrome CLI options:
       // https://peter.sh/experiments/chromium-command-line-switches/
@@ -98,7 +101,7 @@ class Chrome extends Browser {
       final String dir = await generateUserDirectory(installation, useDwarf);
       final List<String> args = <String>[
         '--user-data-dir=$dir',
-        url.toString(),
+        kBlankPageUrl,
         if (!debug)
           '--headless',
         if (isChromeNoSandbox)
@@ -139,6 +142,8 @@ class Chrome extends Browser {
       final Process process =
           await _spawnChromiumProcess(installation.executable, args);
 
+      await setupChromiumTab(url, exceptionCompleter);
+
       remoteDebuggerCompleter.complete(
           getRemoteDebuggerUrl(Uri.parse('http://localhost:$kDevtoolsPort')));
 
@@ -146,10 +151,10 @@ class Chrome extends Browser {
           .then((_) => Directory(dir).deleteSync(recursive: true)));
 
       return process;
-    }), remoteDebuggerCompleter.future);
+    }), remoteDebuggerCompleter.future, exceptionCompleter.future);
   }
 
-  Chrome._(this._process, this.remoteDebuggerUrl);
+  Chrome._(this._process, this.remoteDebuggerUrl, this._onUncaughtException);
 
   static Future<String> generateUserDirectory(
     BrowserInstallation installation,
@@ -211,12 +216,17 @@ class Chrome extends Browser {
 
   final BrowserProcess _process;
 
+  final Future<String> _onUncaughtException;
+
   @override
   final Future<Uri> remoteDebuggerUrl;
 
   @override
   Future<void> get onExit => _process.onExit;
 
+  @override
+  Future<String>? get onUncaughtException => _onUncaughtException;
+
   @override
   Future<void> close() => _process.close();
 
@@ -378,3 +388,28 @@ Future<Uri> getRemoteDebuggerUrl(Uri base) async {
     return base;
   }
 }
+
+Future<void> setupChromiumTab(
+    Uri url, Completer<String> exceptionCompleter) async {
+  final wip.ChromeConnection chromeConnection =
+      wip.ChromeConnection('localhost', kDevtoolsPort);
+  final wip.ChromeTab? chromeTab = await chromeConnection.getTab(
+      (wip.ChromeTab chromeTab) => chromeTab.url == kBlankPageUrl);
+  final wip.WipConnection wipConnection = await chromeTab!.connect();
+
+  await wipConnection.runtime.enable();
+
+  wipConnection.runtime.onExceptionThrown.listen(
+    (wip.ExceptionThrownEvent event) {
+      if (!exceptionCompleter.isCompleted) {
+        final String text = event.exceptionDetails.text;
+        final String? description = event.exceptionDetails.exception?.description;
+        exceptionCompleter.complete('$text: $description');
+      }
+    }
+  );
+
+  await wipConnection.page.enable();
+
+  await wipConnection.page.navigate(url.toString());
+}

engine/src/flutter/lib/web_ui/dev/test_platform.dart

@@ -1059,7 +1059,18 @@ class BrowserManager {
         }
 
         _controllers.add(controller!);
-        return await controller!.suite;
+
+        final List<Future<RunnerSuite>> futures = <Future<RunnerSuite>>[
+          controller!.suite
+        ];
+        if (_browser.onUncaughtException != null) {
+          futures.add(_browser.onUncaughtException!.then<RunnerSuite>(
+              (String error) =>
+                  throw Exception('Exception while loading suite: $error')));
+        }
+
+        final RunnerSuite suite = await Future.any(futures);
+        return suite;
       } catch (_) {
         closeIframe();
         rethrow;