Document how to use pigeon and update to the latest version.

This mainly splits out the test logic from the messages.dart file.

packages/video_player/video_player/CONTRIBUTING.md

@@ -0,0 +1,44 @@
+## Updating pigeon-generated files
+
+If you update files in the pigeons/ directory, run the following
+command in this directory:
+
+```bash
+flutter pub get
+flutter pub run pigeon --dart_null_safety --input pigeons/messages.dart
+(cd ../../../; ./script/incremental_build.sh format --travis --clang-format=clang-format-7)
+```
+
+If you update pigeon itself, temporarily update the pubspec.yaml by
+adding the following to the `dependency_overrides` section, assuming
+you have checked out the `flutter/packages` repo in a sibling
+directory to the `plugins` repo:
+
+```bash
+  pigeon:
+    path:
+      ../../../../packages/packages/pigeon/
+```
+
+Then, run the commands above. When you run `pub get` it should warn
+you that you're using an override. If you do this, you will need to
+publish pigeon before you can land the updates to this package, since
+the CI tests run the analysis using latest published version of
+pigeon, not your version or the version on master.
+
+In either case, the configuration will be obtained automatically from
+the `pigeons/messages.dart` file (see `configurePigeon` at the bottom
+of that file).
+
+While contributing, you may also want to set the following dependency
+overrides:
+
+```yaml
+dependency_overrides:
+  video_player_platform_interface:
+    path:
+      ../video_player_platform_interface
+  video_player_web:
+    path:
+      ../video_player_web
+```

packages/video_player/video_player/android/src/main/java/io/flutter/plugins/videoplayer/Messages.java

@@ -1,4 +1,4 @@
-// Autogenerated from Pigeon (v0.1.12), do not edit directly.
+// Autogenerated from Pigeon (v0.1.17), do not edit directly.
 // See also: https://pub.dev/packages/pigeon
 
 package io.flutter.plugins.videoplayer;

packages/video_player/video_player/ios/Classes/messages.h

@@ -1,4 +1,4 @@
-// Autogenerated from Pigeon (v0.1.12), do not edit directly.
+// Autogenerated from Pigeon (v0.1.17), do not edit directly.
 // See also: https://pub.dev/packages/pigeon
 #import <Foundation/Foundation.h>
 @protocol FlutterBinaryMessenger;

packages/video_player/video_player/ios/Classes/messages.m

@@ -1,4 +1,4 @@
-// Autogenerated from Pigeon (v0.1.12), do not edit directly.
+// Autogenerated from Pigeon (v0.1.17), do not edit directly.
 // See also: https://pub.dev/packages/pigeon
 #import "messages.h"
 #import <Flutter/Flutter.h>
@@ -7,7 +7,6 @@
 #error File requires ARC to be enabled.
 #endif
 
-#ifndef __clang_analyzer__
 static NSDictionary *wrapResult(NSDictionary *result, FlutterError *error) {
   NSDictionary *errorDict = (NSDictionary *)[NSNull null];
   if (error) {
@@ -362,4 +361,3 @@ void FLTVideoPlayerApiSetup(id<FlutterBinaryMessenger> binaryMessenger, id<FLTVi
     }
   }
 }
-#endif

packages/video_player/video_player/pigeons/messages.dart

@@ -54,6 +54,7 @@ abstract class VideoPlayerApi {
 
 void configurePigeon(PigeonOptions opts) {
   opts.dartOut = '../video_player_platform_interface/lib/messages.dart';
+  opts.dartTestOut = '../video_player_platform_interface/lib/test.dart';
   opts.objcHeaderOut = 'ios/Classes/messages.h';
   opts.objcSourceOut = 'ios/Classes/messages.m';
   opts.objcOptions.prefix = 'FLT';

packages/video_player/video_player/pubspec.yaml

@@ -1,9 +1,6 @@
 name: video_player
 description: Flutter plugin for displaying inline video with other Flutter
   widgets on Android, iOS, and web.
-# 0.10.y+z is compatible with 1.0.0, if you land a breaking change bump
-# the version to 2.0.0.
-# See more details: https://github.com/flutter/flutter/wiki/Package-migration-to-1.0.0
 version: 2.0.0-nullsafety.6
 homepage: https://github.com/flutter/plugins/tree/master/packages/video_player/video_player
 
@@ -31,12 +28,12 @@ dependencies:
 
   flutter:
     sdk: flutter
-
-dev_dependencies:
   flutter_test:
     sdk: flutter
+
+dev_dependencies:
   pedantic: ^1.10.0-nullsafety.1
-  pigeon: 0.1.7
+  pigeon: ^0.1.17
 
 environment:
   sdk: ">=2.12.0-0 <3.0.0"

packages/video_player/video_player/test/video_player_test.dart

@@ -12,6 +12,7 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:video_player/video_player.dart';
 import 'package:video_player_platform_interface/messages.dart';
+import 'package:video_player_platform_interface/test.dart';
 import 'package:video_player_platform_interface/video_player_platform_interface.dart';
 
 class FakeController extends ValueNotifier<VideoPlayerValue>