Update note about web and plugins (#6399)

The note at the top of the page about federated plugins is confusing, since federation and communication method are actually completely orthogonal, and this page isn't actually about plugins in the first place. This removes discussion of federation and plugins entirely, and instead talks about web implementation of platform-specific code directly.

Author: stuartmorgan-g

src/docs/development/platform-integration/platform-channels.md

@@ -9,10 +9,9 @@ platform-specific functionality is available through existing packages;
 see [using packages][].
 
 {{site.alert.note}}
-  The information in this page is valid for most
-  plugins, except for any web plugin implemented
-  as a [federated plugin][]. Federated plugins
-  don't use platform channels.
+  The information in this page is valid for most platforms,
+  but platform-specific code for the web generally uses
+  [JS interoperability][] or the [`dart:html` library][] instead.
 {{site.alert.end}}
 
 Flutter uses a flexible system that allows you to call
@@ -785,13 +784,14 @@ DispatchQueue.main.async {
 [`BinaryCodec`]: {{site.api}}/flutter/services/BinaryCodec-class.html
 [block]: {{site.apple-dev}}/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithBlocks/WorkingwithBlocks.html
 [`cloud_firestore`]: {{site.github}}/FirebaseExtended/flutterfire/blob/master/packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/method_channel/utils/firestore_message_codec.dart
+[`dart:html` library]: {{site.dart.api}}/dart-html/dart-html-library.html
 [defaultTargetPlatform]: {{site.api}}/flutter/foundation/defaultTargetPlatform.html
 [developing packages]: /docs/development/packages-and-plugins/developing-packages
 [plugins]: /docs/development/packages-and-plugins/developing-packages#plugin
 [dispatch queue]: {{site.apple-dev}}/documentation/dispatch/dispatchqueue
 [`/examples/platform_channel/`]: {{site.github}}/flutter/flutter/tree/master/examples/platform_channel
 [`/examples/platform_channel_swift/`]: {{site.github}}/flutter/flutter/tree/master/examples/platform_channel_swift
-[federated plugin]: /docs/development/packages-and-plugins/developing-packages#federated-plugins
+[JS interoperability]: {{site.dart-site}}/web/js-interop
 [`JSONMessageCodec`]: {{site.api}}/flutter/services/JSONMessageCodec-class.html
 [`MethodChannel`]: {{site.api}}/flutter/services/MethodChannel-class.html
 [`MethodChannelAndroid`]: {{site.api}}/javadoc/io/flutter/plugin/common/MethodChannel.html