Sync async2 (#17964)

Enable --sync-async in Flutter.
Fixes #16801 

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 +14,10 @@ class TestTestBinding extends AutomatedTestWidgetsFlutterBinding {
 
 Future<Null> guardedHelper(WidgetTester tester) {
   return TestAsyncUtils.guard(() async {
-    await tester.pumpWidget(const Text('Hello'));
+    await tester.pumpWidget(const Directionality(
+      textDirection: TextDirection.ltr,
+      child: const Text('Hello'),
+    ));
   });
 }
 

packages/flutter_test/lib/src/binding.dart

@@ -490,6 +490,12 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   }
 
   Future<Null> _runTestBody(Future<Null> testBody(), VoidCallback invariantTester) async {
+    // Delay this function by a microtask.
+    // Otherwise it will open a scope immediately, which is then open when
+    // the `asyncBarrier` is invoked. The `asyncBarrier` is immediately
+    // following the call to `testZone.runBinary(_runTestBody)`, so delaying
+    // by one microtask is enough to ensure that the timing is correct.
+    await new Future<Null>.microtask(() {});
     assert(inTest);
 
     runApp(new Container(key: new UniqueKey(), child: _preTestMessage)); // Reset the tree to a known state.
@@ -767,6 +773,12 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     });
 
     return new Future<Null>.microtask(() async {
+      // Run all queued microtasks.
+      await new Future<Null>.microtask(() {});
+      // When the test had an exception, the test-framework already
+      // ran the teardown functions, removing the _fakeAsync function.
+      if (_fakeAsync == null)
+        return null;
       // Resolve interplay between fake async and real async calls.
       _fakeAsync.flushMicrotasks();
       while (_pendingAsyncTasks != null) {
@@ -983,6 +995,13 @@ class LiveTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
 
   @override
   void handleBeginFrame(Duration rawTimeStamp) {
+    // Don't run this function when `handleBeginFrame` was invoked
+    // immediately before without a call of `handleDrawFrame` in between.
+    // TODO(floitsch): Remove this line when the spurious calls from the
+    //                 framework don't happen anymore. See
+    //                 https://github.com/flutter/flutter/issues/17963
+    if (_doDrawThisFrame != null)
+      return;
     assert(_doDrawThisFrame == null);
     if (_expectingFrame ||
         (framePolicy == LiveTestWidgetsFlutterBindingFramePolicy.fullyLive) ||
@@ -997,6 +1016,13 @@ class LiveTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
 
   @override
   void handleDrawFrame() {
+    // Don't run this function when `handleBeginFrame` wasn't invoked
+    // immediately before.
+    // TODO(floitsch): Remove this line when the spurious calls from the
+    //                 framework don't happen anymore. See
+    //                 https://github.com/flutter/flutter/issues/17963
+    if (_doDrawThisFrame == null)
+      return;
     assert(_doDrawThisFrame != null);
     if (_doDrawThisFrame)
       super.handleDrawFrame();

packages/flutter_tools/lib/src/compile.dart

@@ -120,6 +120,7 @@ class KernelCompiler {
       '--sdk-root',
       sdkRoot,
       '--strong',
+      '--sync-async',
       '--target=flutter',
     ];
     if (trackWidgetCreation)
@@ -252,6 +253,7 @@ class ResidentCompiler {
       _sdkRoot,
       '--incremental',
       '--strong',
+      '--sync-async',
       '--target=flutter',
     ];
     if (outputPath != null) {