Fix microbenchmarks tests. (#9782)

dev/benchmarks/microbenchmarks/lib/stocks/build_bench.dart

@@ -44,7 +44,11 @@ Future<Null> main() async {
       // artificially high load to find out how much CPU each frame takes.
       // This differs from normal benchmarks which might look at how many
       // frames are missed, etc.
-      ui.window.onBeginFrame(new Duration(milliseconds: iterations * 16));
+      // We use Timer.run to ensure there's a microtask flush in between
+      // the two calls below.
+      Timer.run(() { ui.window.onBeginFrame(new Duration(milliseconds: iterations * 16)); });
+      Timer.run(() { ui.window.onDrawFrame(); });
+      await tester.idle(); // wait until the frame has run
       iterations += 1;
     }
     watch.stop();

dev/benchmarks/microbenchmarks/lib/stocks/layout_bench.dart

@@ -45,7 +45,11 @@ Future<Null> main() async {
       // artificially high load to find out how much CPU each frame takes.
       // This differs from normal benchmarks which might look at how many
       // frames are missed, etc.
-      ui.window.onBeginFrame(new Duration(milliseconds: iterations * 16));
+      // We use Timer.run to ensure there's a microtask flush in between
+      // the two calls below.
+      Timer.run(() { ui.window.onBeginFrame(new Duration(milliseconds: iterations * 16)); });
+      Timer.run(() { ui.window.onDrawFrame(); });
+      await tester.idle(); // wait until the frame has run
       iterations += 1;
     }
     watch.stop();

dev/devicelab/lib/tasks/microbenchmarks.dart

@@ -29,9 +29,8 @@ TaskFunction createMicrobenchmarkTask() {
         final Directory appDir = dir(
             path.join(flutterDirectory.path, 'dev/benchmarks/microbenchmarks'));
         final Process flutterProcess = await inDirectory(appDir, () async {
-          if (deviceOperatingSystem == DeviceOperatingSystem.ios) {
+          if (deviceOperatingSystem == DeviceOperatingSystem.ios)
             await prepareProvisioningCertificates(appDir.path);
-          }
           return await _startFlutter(
             options: <String>[
               '--profile',