Feature/callback messages

[pchampio]

Golang impl of flutter/engine@6aad139

I chose to break the plugin api:

@@ plugin/method-channel.go
-  _, err := MethodChannel.InvokeMethod("popRoute",nil)
+  err := MethodChannel.InvokeMethod("popRoute",nil)

@@ plugin/basic-message-channel.go
-  _, err := BasicMessageChannel.Send("xxx")
+  err = BasicMessageChannel.Send("xxx")

Why?

Peoples are used to not return result in their flutter setMethodCallHandler/setMessageHandler.
example:

  platform_channel
      .setMethodCallHandler((MethodCall methodCall) async {
          // do sutff

          // rarely calls:
          // return "42"; 
  });

With a InvokeMethod that expect a reply, the embedder will wait forever, (not blocking the UI but the behavior isn't normal).

The uses of InvokeMethodWithReply and SendWithReply is enough to let the user know he has to return a value in the flutter handler.

*WithReply is also used in the flutter/engine desktop shell, users will not get lost.

[pchampio]

I have checked on github, the changes in the plugin api shouldn't affect anyone.