Update plugin template to comply with new type and style rules (#17840)

packages/flutter_tools/templates/create/android-java.tmpl/app/src/main/java/androidIdentifier/MainActivity.java.tmpl

 package {{androidIdentifier}};
 
 import android.os.Bundle;
-
 import io.flutter.app.FlutterActivity;
 import io.flutter.plugins.GeneratedPluginRegistrant;
 

packages/flutter_tools/templates/create/ios-objc.tmpl/Runner/AppDelegate.h

-#import <UIKit/UIKit.h>
 #import <Flutter/Flutter.h>
+#import <UIKit/UIKit.h>
 
 @interface AppDelegate : FlutterAppDelegate
 

packages/flutter_tools/templates/create/ios-objc.tmpl/Runner/AppDelegate.m

@@ -3,7 +3,8 @@
 
 @implementation AppDelegate
 
-- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
+- (BOOL)application:(UIApplication *)application
+    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
   [GeneratedPluginRegistrant registerWithRegistry:self];
   // Override point for customization after application launch.
   return [super application:application didFinishLaunchingWithOptions:launchOptions];

packages/flutter_tools/templates/create/ios-objc.tmpl/Runner/main.m

-#import <UIKit/UIKit.h>
 #import <Flutter/Flutter.h>
+#import <UIKit/UIKit.h>
 #import "AppDelegate.h"
 
-int main(int argc, char * argv[]) {
+int main(int argc, char* argv[]) {
   @autoreleasepool {
     return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
   }

packages/flutter_tools/templates/create/lib/main.dart.tmpl

@@ -3,6 +3,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_driver/driver_extension.dart';
 {{/withDriverTest}}
 {{#withPluginHook}}
+import 'dart:async';
+
 import 'package:flutter/services.dart';
 import 'package:{{pluginProjectName}}/{{pluginProjectName}}.dart';
 {{/withPluginHook}}
@@ -136,13 +138,13 @@ class _MyAppState extends State<MyApp> {
   String _platformVersion = 'Unknown';
 
   @override
-  initState() {
+  void initState() {
     super.initState();
     initPlatformState();
   }
 
   // Platform messages are asynchronous, so we initialize in an async method.
-  initPlatformState() async {
+  Future<void> initPlatformState() async {
     String platformVersion;
     // Platform messages may fail, so we use a try/catch PlatformException.
     try {
@@ -154,8 +156,7 @@ class _MyAppState extends State<MyApp> {
     // If the widget was removed from the tree while the asynchronous platform
     // message was in flight, we want to discard the reply rather than calling
     // setState to update our non-existent appearance.
-    if (!mounted)
-      return;
+    if (!mounted) return;
 
     setState(() {
       _platformVersion = platformVersion;
@@ -167,7 +168,7 @@ class _MyAppState extends State<MyApp> {
     return new MaterialApp(
       home: new Scaffold(
         appBar: new AppBar(
-          title: new Text('Plugin example app'),
+          title: const Text('Plugin example app'),
         ),
         body: new Center(
           child: new Text('Running on: $_platformVersion\n'),

packages/flutter_tools/templates/plugin/CHANGELOG.md.tmpl

-## [0.0.1] - TODO: Add release date.
+## 0.0.1
 
 * TODO: Describe initial release.

packages/flutter_tools/templates/plugin/android-java.tmpl/src/main/java/androidIdentifier/pluginClass.java.tmpl

 package {{androidIdentifier}};
 
+import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.PluginRegistry.Registrar;
 
-/**
- * {{pluginClass}}
- */
+/** {{pluginClass}} */
 public class {{pluginClass}} implements MethodCallHandler {
-  /**
-   * Plugin registration.
-   */
+  /** Plugin registration. */
   public static void registerWith(Registrar registrar) {
     final MethodChannel channel = new MethodChannel(registrar.messenger(), "{{projectName}}");
     channel.setMethodCallHandler(new {{pluginClass}}());