feat(platform_interface): create my_plugin_platform_interface (#1)

src/my_plugin_platform_interface/lib/my_plugin_platform_interface.dart

@@ -0,0 +1,33 @@
+import 'package:my_plugin_platform_interface/src/method_channel_my_plugin.dart';
+import 'package:plugin_platform_interface/plugin_platform_interface.dart';
+
+/// The interface that implementations of my_plugin must implement.
+///
+/// Platform implementations should extend this class
+/// rather than implement it as `MyPlugin`.
+/// Extending this class (using `extends`) ensures that the subclass will get
+/// the default implementation, while platform implementations that `implements`
+///  this interface will be broken by newly added [MyPluginPlatform] methods.
+abstract class MyPluginPlatform extends PlatformInterface {
+  /// Constructs a MyPluginPlatform.
+  MyPluginPlatform() : super(token: _token);
+
+  static final Object _token = Object();
+
+  static MyPluginPlatform _instance = MethodChannelMyPlugin();
+
+  /// The default instance of [MyPluginPlatform] to use.
+  ///
+  /// Defaults to [MethodChannelMyPlugin].
+  static MyPluginPlatform get instance => _instance;
+
+  /// Platform-specific plugins should set this with their own platform-specific
+  /// class that extends [MyPluginPlatform] when they register themselves.
+  static set instance(MyPluginPlatform instance) {
+    PlatformInterface.verify(instance, _token);
+    _instance = instance;
+  }
+
+  /// Return the current platform name.
+  Future<String?> getPlatformName();
+}

src/my_plugin_platform_interface/lib/src/method_channel_my_plugin.dart

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+import 'package:flutter/foundation.dart' show visibleForTesting;
+import 'package:flutter/services.dart';
+import 'package:my_plugin_platform_interface/my_plugin_platform_interface.dart';
+
+/// An implementation of [MyPluginPlatform] that uses method channels.
+class MethodChannelMyPlugin extends MyPluginPlatform {
+  /// The method channel used to interact with the native platform.
+  @visibleForTesting
+  final methodChannel = const MethodChannel('my_plugin');
+
+  @override
+  Future<String?> getPlatformName() {
+    return methodChannel.invokeMethod<String>('getPlatformName');
+  }
+}