Add --replay-from argument to Flutter tools (#7146)

This argument will enable mocking of os-layer process invocations,
where the mock behavior will come from replaying a previously-
recorded set of invocations. At the point of process invocation,
the key metadata for the invocation will be looked up in the
recording's manifest, and iff a matching record exists in the
manifest, the process will be mocked out with data derived from
the corresponding recorded process (e.g. stdout, stderr, exit code).

packages/flutter_tools/lib/src/ios/simulators.dart

diff --git a/packages/flutter_tools/lib/src/ios/simulators.dart b/packages/flutter_tools/lib/src/ios/simulators.dart
index a0e5a570..bc48e69 100644
--- a/packages/flutter_tools/lib/src/ios/simulators.dart
+++ b/packages/flutter_tools/lib/src/ios/simulators.dart
@@ -422,7 +422,8 @@
 
       try {
         await _setupUpdatedApplicationBundle(app);
-      } on ToolExit {
+      } on ToolExit catch (e) {
+        printError(e.message);
         return new LaunchResult.failed();
       }
     } else {
@@ -507,7 +508,7 @@
     // Step 2: Assert that the Xcode project was successfully built.
     IOSApp iosApp = app;
     Directory bundle = new Directory(iosApp.simulatorBundlePath);
-    bool bundleExists = await bundle.exists();
+    bool bundleExists = bundle.existsSync();
     if (!bundleExists)
       throwToolExit('Could not find the built application bundle at ${bundle.path}.');