added test that passes before this change, and fails after it

Author: gaaclarke

shell/platform/darwin/ios/framework/Source/FlutterEngineTest.mm

@@ -218,4 +218,25 @@ - (void)testDeallocNotification {
   [center removeObserver:observer];
 }
 
+- (void)testSetHandlerAfterRun {
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"foobar"];
+  NSObject<FlutterPluginRegistrar>* registrar = [engine registrarForPlugin:@"foo"];
+  XCTestExpectation* gotMessage = [self expectationWithDescription:@"gotMessage"];
+  fml::AutoResetWaitableEvent latch;
+  [engine run];
+  flutter::Shell& shell = engine.shell;
+  engine.shell.GetTaskRunners().GetUITaskRunner()->PostTask([&latch, &shell] {
+    flutter::Engine::Delegate& delegate = shell;
+    auto message = std::make_unique<flutter::PlatformMessage>("foo", nullptr);
+    delegate.OnEngineHandlePlatformMessage(std::move(message));
+    latch.Signal();
+  });
+  latch.Wait();
+  [registrar.messenger setMessageHandlerOnChannel:@"foo"
+                             binaryMessageHandler:^(NSData* message, FlutterBinaryReply reply) {
+                               [gotMessage fulfill];
+                             }];
+  [self waitForExpectationsWithTimeout:1 handler:nil];
+}
+
 @end

shell/platform/darwin/ios/framework/Source/FlutterEngine_Test.h

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#import "flutter/shell/common/shell.h"
 #import "flutter/shell/platform/darwin/ios/framework/Headers/FlutterEngine.h"
 #import "flutter/shell/platform/darwin/ios/rendering_api_selection.h"
 
@@ -15,6 +16,7 @@ class ThreadHost;
 
 // Category to add test-only visibility.
 @interface FlutterEngine (Test) <FlutterBinaryMessenger>
+- (flutter::Shell&)shell;
 - (void)setBinaryMessenger:(FlutterBinaryMessengerRelay*)binaryMessenger;
 - (flutter::IOSRenderingAPI)platformViewsRenderingAPI;
 - (void)waitForFirstFrame:(NSTimeInterval)timeout callback:(void (^)(BOOL didTimeout))callback;