Roll sync async engine (#18454)

* Roll the engine.

This engine includes a VM that defaults to sync-async.

flutter/engine@c8eeee4 Roll the Dart VM.

bin/internal/engine.version

-c3976b3c7183f479717bffed3f640fb92afbd3dc
+c8eeee432bb362d657186faa00b3de7c468a9062

dev/automated_tests/flutter_test/exception_handling_expectation.txt

@@ -43,7 +43,10 @@ Who lives, who dies, who tells your story\?
 
 When the exception was thrown, this was the stack:
 #[0-9]+ +main.<anonymous closure> \(.+[/\\]dev[/\\]automated_tests[/\\]flutter_test[/\\]exception_handling_test\.dart:16:9\)
-#[0-9]+ +main.<anonymous closure> \(.+[/\\]dev[/\\]automated_tests[/\\]flutter_test[/\\]exception_handling_test\.dart:15:77\)
+#[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]binding.dart:[0-9]+:[0-9]+\)
+#[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]binding.dart:[0-9]+:[0-9]+\)
+#[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]binding.dart:[0-9]+:[0-9]+\)
+#[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]binding.dart:[0-9]+:[0-9]+\)
 #[0-9]+ +.+ \(package:flutter_test[/\\]src[/\\]widget_tester\.dart:[0-9]+:[0-9]+\)
 <<skip until matching line>>
 ^\(elided [0-9]+ .+\)$

dev/automated_tests/flutter_test/test_async_utils_guarded_test.dart

@@ -14,7 +14,7 @@ class TestTestBinding extends AutomatedTestWidgetsFlutterBinding {
 
 Future<Null> guardedHelper(WidgetTester tester) {
   return TestAsyncUtils.guard(() async {
-    await tester.pumpWidget(const Text('Hello'));
+    await tester.pumpWidget(const Text('Hello', textDirection: TextDirection.ltr));
   });
 }
 

packages/flutter/test/engine/dart_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:test/test.dart';
+
+/// Verifies Dart semantics governed by flags set by Flutter tooling.
+void main() {
+  group('Async', () {
+    String greeting = 'hello';
+    Future<void> changeGreeting() async {
+      greeting += ' 1';
+      await new Future<void>.value(null);
+      greeting += ' 2';
+    }
+    test('execution of async method starts synchronously', () async {
+      expect(greeting, 'hello');
+      final Future<void> future = changeGreeting();
+      expect(greeting, 'hello 1');
+      await future;
+      expect(greeting, 'hello 1 2');
+    });
+  });
+}

packages/flutter_test/lib/src/binding.dart

@@ -502,7 +502,6 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     testZone.runBinary(_runTestBody, testBody, invariantTester)
       .whenComplete(testCompletionHandler);
     timeout?.catchError(handleUncaughtError);
-    asyncBarrier(); // When using AutomatedTestWidgetsFlutterBinding, this flushes the microtasks.
     return testCompleter.future;
   }
 
@@ -532,6 +531,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     }
 
     assert(inTest);
+    asyncBarrier(); // When using AutomatedTestWidgetsFlutterBinding, this flushes the microtasks.
     return null;
   }
 
@@ -848,22 +848,23 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     });
 
     return new Future<Null>.microtask(() async {
+      // testBodyResult is a Future that was created in the Zone of the
+      // fakeAsync. This means that if we await it here, it will register a
+      // microtask to handle the future _in the fake async zone_. We avoid this
+      // by calling '.then' in the current zone. While flushing the microtasks
+      // of the fake-zone below, the new future will be completed and can then
+      // be used without fakeAsync.
+      final Future<Null> resultFuture = testBodyResult.then<Null>((_) {
+        // Do nothing.
+      });
+
       // Resolve interplay between fake async and real async calls.
       fakeAsync.flushMicrotasks();
       while (_pendingAsyncTasks != null) {
         await _pendingAsyncTasks.future;
         fakeAsync.flushMicrotasks();
       }
-
-      // If we get here and fakeAsync != _currentFakeAsync, then the test
-      // probably timed out.
-
-      // testBodyResult is a Future that was created in the Zone of the
-      // fakeAsync. This means that if we await it here, it will register a
-      // microtask to handle the future _in the fake async zone_. We avoid this
-      // by returning the wrapped microtask future that we've created _outside_
-      // the fake async zone.
-      return testBodyResult;
+      return resultFuture;
     });
   }