Catch MissingPortFile exception (#43573)

packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart

@@ -249,6 +249,10 @@ class ResidentWebRunner extends ResidentRunner {
         'This can happen if the websocket connection used by the web tooling is '
         'unabled to correctly establish a connection, for example due to a firewall.'
       );
+     } on MissingPortFile {
+      throwToolExit(
+        'Failed to connect to build daemon.\nThe daemon either failed to '
+        'start or was killed by another process.');
     } on SocketException catch (err) {
       throwToolExit(err.toString());
     } finally {

packages/flutter_tools/test/general.shard/resident_web_runner_test.dart

@@ -677,6 +677,25 @@ void main() {
     await expectation;
   }));
 
+  test('Successfully turns MissingPortFile error into ToolExit', () => testbed.run(() async {
+    _setupMocks();
+    final Completer<DebugConnectionInfo> connectionInfoCompleter = Completer<DebugConnectionInfo>();
+    final Completer<void> unhandledErrorCompleter = Completer<void>();
+    when(mockWebFs.connect(any)).thenAnswer((Invocation _) async {
+      unawaited(unhandledErrorCompleter.future.then((void value) {
+        throw MissingPortFile();
+      }));
+      return ConnectionResult(mockAppConnection, mockDebugConnection);
+    });
+
+    final Future<void> expectation = expectLater(() => residentWebRunner.run(
+      connectionInfoCompleter: connectionInfoCompleter,
+    ), throwsA(isInstanceOf<ToolExit>()));
+
+    unhandledErrorCompleter.complete();
+    await expectation;
+  }));
+
   test('Rethrows unknown exception type from web tooling', () => testbed.run(() async {
     _setupMocks();
     final DelegateLogger delegateLogger = logger;