Do not swallow exceptions in guarded functions. (#9064)

packages/flutter/test/material/refresh_indicator_test.dart

@@ -88,7 +88,7 @@ void main() {
       ),
     );
 
-    await tester.fling(find.text('X'), const Offset(0.0, -300.0), 1000.0);
+    await tester.fling(find.text('X'), const Offset(0.0, 300.0), 1000.0);
     await tester.pump();
     await tester.pump(const Duration(seconds: 1));
     await tester.pump(const Duration(seconds: 1));

packages/flutter_test/lib/src/controller.dart

@@ -380,6 +380,7 @@ class WidgetController {
   Future<Null> dragFrom(Point startLocation, Offset offset, { int pointer }) {
     return TestAsyncUtils.guard(() async {
       final TestGesture gesture = await startGesture(startLocation, pointer: pointer);
+      assert(gesture != null);
       await gesture.moveBy(offset);
       await gesture.up();
       return null;

packages/flutter_test/lib/src/test_async_utils.dart

@@ -67,7 +67,7 @@ class TestAsyncUtils {
     final _AsyncScope scope = new _AsyncScope(StackTrace.current, zone);
     _scopeStack.add(scope);
     final Future<Null> result = scope.zone.run(body);
-    void completionHandler(dynamic error, StackTrace stack) {
+    Future<Null> completionHandler(dynamic error, StackTrace stack) {
       assert(_scopeStack.isNotEmpty);
       assert(_scopeStack.contains(scope));
       bool leaked = false;
@@ -101,10 +101,13 @@ class TestAsyncUtils {
         }
         throw new FlutterError(message.toString().trimRight());
       }
+      if (error != null)
+        return new Future<Null>.error(error, stack);
+      return new Future<Null>.value(null);
     }
     return result.then<Null>(
       (Null value) {
-        completionHandler(null, null);
+        return completionHandler(null, null);
       },
       onError: completionHandler
     );

packages/flutter_test/lib/src/test_pointer.dart

@@ -140,9 +140,8 @@ class TestGesture {
   }) async {
     assert(hitTester != null);
     assert(dispatcher != null);
-    final Completer<TestGesture> completer = new Completer<TestGesture>();
     TestGesture result;
-    TestAsyncUtils.guard(() async {
+    return TestAsyncUtils.guard(() async {
       // dispatch down event
       final HitTestResult hitTestResult = hitTester(downLocation);
       final TestPointer testPointer = new TestPointer(pointer);
@@ -151,10 +150,11 @@ class TestGesture {
       // create a TestGesture
       result = new TestGesture._(dispatcher, hitTestResult, testPointer);
       return null;
-    }).whenComplete(() {
-      completer.complete(result);
+    }).then<TestGesture>((Null value) {
+      return result;
+    }, onError: (dynamic error, StackTrace stack) {
+      return new Future<TestGesture>.error(error, stack);
     });
-    return completer.future;
   }
 
   final EventDispatcher _dispatcher;

packages/flutter_test/test/test_async_utils_test.dart

@@ -27,13 +27,9 @@ class TestAPISubclass extends TestAPI {
   }
 }
 
-Future<Null> helperFunction(WidgetTester tester) async {
-  await tester.pump();
-}
-
-Future<Null> guardedHelper(WidgetTester tester) {
+Future<Null> _guardedThrower() {
   return TestAsyncUtils.guard(() async {
-    await tester.pumpWidget(new Text('Hello'));
+    throw 'Hello';
   });
 }
 
@@ -174,5 +170,14 @@ void main() {
     await f1;
   });
 
+  testWidgets('TestAsyncUtils - guard body can throw', (WidgetTester tester) async {
+    try {
+      await _guardedThrower();
+      expect(false, true); // _guardedThrower should throw and we shouldn't reach here
+    } on String catch (s) {
+      expect(s, 'Hello');
+    }
+  });
+
   // see also dev/manual_tests/test_data which contains tests run by the flutter_tools tests for 'flutter test'
 }