[webview_flutter_web] Explain web registration (#4668)

The README instructions currently use the standard description of using
an unendorsed implementation of a federated package, which assumes
auto-registration. Because `webview_flutter_web` doesn't use
autoregistration due to the current structural issues in
`webview_flutter`'s federation, those instructions aren't complete.

This adds an explanation of registering the implementation, including an
example of using conditional imports for multi-platform projects.

packages/webview_flutter/webview_flutter_web/CHANGELOG.md

@@ -1,4 +1,7 @@
-## 0.1.0
+## 0.1.0+1
+
+* Adds an explanation of registering the implementation in the README.
 
-*  First web implementation for webview_flutter
+## 0.1.0
 
+* First web implementation for webview_flutter

packages/webview_flutter/webview_flutter_web/README.md

@@ -14,7 +14,65 @@ Nothing else is currently supported.
 
 ## Usage
 
-### Depend on the package
+This package is not an endorsed implementation of the `webview_flutter` plugin
+yet, so it currently requires extra setup to use:
 
-This package is not an endorsed implementation of the `webview_flutter` plugin yet, so you'll need to
-[add it explicitly](https://pub.dev/packages/webview_flutter_web/install).
+* [Add this package](https://pub.dev/packages/webview_flutter_web/install)
+  as an explicit dependency of your project, in addition to depending on
+  `webview_flutter`.
+* Register `WebWebViewPlatform` as the `WebView.platform` before creating a
+  `WebView`. See below for examples.
+
+Once those steps below are complete, the APIs from `webview_flutter` listed
+above can be used as normal on web.
+
+### Registering the implementation
+
+Before creating a `WebView` (for instance, at the start of `main`), you will
+need to register the web implementation.
+
+#### Web-only project example
+
+```dart
+...
+import 'package:webview_flutter/webview_flutter.dart';
+import 'package:webview_flutter_web/webview_flutter_web.dart';
+
+main() {
+  WebView.platform = WebWebViewPlatform();
+  ...
+```
+
+#### Multi-platform project example
+
+If your project supports platforms other than web, you will need to use a
+conditional import to avoid directly including `webview_flutter_web.dart` on
+non-web platforms. For example:
+
+`register_web_webview.dart`:
+```dart
+import 'package:webview_flutter/webview_flutter.dart';
+import 'package:webview_flutter_web/webview_flutter_web.dart';
+
+void registerWebViewWebImplementation() {
+  WebView.platform = WebWebViewPlatform();
+}
+```
+
+`register_web_webview_stub.dart`:
+```dart
+void registerWebViewWebImplementation() {
+  // No-op.
+}
+```
+
+`main.dart`:
+```dart
+...
+import 'register_web_webview_stub.dart'
+    if (dart.library.html) 'register_web.dart';
+
+main() {
+  registerWebViewWebImplementation();
+  ...
+```

packages/webview_flutter/webview_flutter_web/pubspec.yaml

@@ -2,7 +2,7 @@ name: webview_flutter_web
 description: A Flutter plugin that provides a WebView widget on web.
 repository: https://github.com/flutter/plugins/tree/master/packages/webview_flutter/webview_flutter_web
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview%22
-version: 0.1.0
+version: 0.1.0+1
 
 environment:
   sdk: ">=2.14.0 <3.0.0"