[web] Fix 'felt test' watch mode (flutter/engine#36423)

engine/src/flutter/lib/web_ui/dev/steps/run_tests_step.dart

@@ -38,7 +38,7 @@ class RunTestsStep implements PipelineStep {
     required this.requireSkiaGold,
     this.testFiles,
     required this.overridePathToCanvasKit,
-  }) : _browserEnvironment = getBrowserEnvironment(browserName);
+  });
 
   final String browserName;
   final List<FilePath>? testFiles;
@@ -49,8 +49,6 @@ class RunTestsStep implements PipelineStep {
   /// Require Skia Gold to be available and reachable.
   final bool requireSkiaGold;
 
-  final BrowserEnvironment _browserEnvironment;
-
   @override
   String get description => 'run_tests';
 
@@ -63,14 +61,16 @@ class RunTestsStep implements PipelineStep {
   @override
   Future<void> run() async {
     await _prepareTestResultsDirectory();
-    await _browserEnvironment.prepare();
+
+    final BrowserEnvironment browserEnvironment = getBrowserEnvironment(browserName);
+    await browserEnvironment.prepare();
 
     final SkiaGoldClient? skiaClient = await _createSkiaClient();
     final List<FilePath> testFiles = this.testFiles ?? findAllTests();
 
     await _runTestBatch(
       testFiles: testFiles,
-      browserEnvironment: _browserEnvironment,
+      browserEnvironment: browserEnvironment,
       expectFailure: false,
       isDebug: isDebug,
       doUpdateScreenshotGoldens: doUpdateScreenshotGoldens,
@@ -78,7 +78,7 @@ class RunTestsStep implements PipelineStep {
       overridePathToCanvasKit: overridePathToCanvasKit,
     );
 
-    await _browserEnvironment.cleanup();
+    await browserEnvironment.cleanup();
 
     if (io.exitCode != 0) {
       throw ToolExit('Some tests failed');

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

@@ -141,12 +141,30 @@ class TestCommand extends Command<bool> with ArgUtils<bool> {
         overridePathToCanvasKit: overridePathToCanvasKit,
       ),
     ]);
-    await testPipeline.run();
+
+    try {
+      await testPipeline.run();
+      if (isWatchMode) {
+        print('');
+        print('Initial test succeeded!');
+      }
+    } catch(error, stackTrace) {
+      if (isWatchMode) {
+        // The error is printed but not rethrown in watch mode because
+        // failures are expected. The idea is that the developer corrects the
+        // error, saves the file, and the pipeline reruns.
+        print('');
+        print('Initial test failed!\n');
+        print(error);
+        print(stackTrace);
+      } else {
+        rethrow;
+      }
+    }
 
     if (isWatchMode) {
       final FilePath dir = FilePath.fromWebUi('');
       print('');
-      print('Initial test run is done!');
       print(
           'Watching ${dir.relativeToCwd}/lib and ${dir.relativeToCwd}/test to re-run tests');
       print('');