update repo to be forward compatible with shelf_web_socket v3.0 (#161101)

- update the engine and flutter_tools to be forward compatible with the
upcoming shelf_web_socket v3.0

*List which issues are fixed by this PR. You must list at least one
issue. An issue is not required if the PR fixes something trivial like a
typo.*

- https://github.com/dart-lang/shelf/issues/457
- https://github.com/dart-lang/shelf/pull/463

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/lib/web_ui/dev/test_platform.dart

@@ -619,7 +619,11 @@ class BrowserPlatform extends PlatformPlugin {
     }
 
     final Completer<WebSocketChannel> completer = Completer<WebSocketChannel>.sync();
-    final String path = _webSocketHandler.create(webSocketHandler(completer.complete));
+    final String path = _webSocketHandler.create(
+      webSocketHandler((WebSocketChannel webSocket, _) {
+        completer.complete(webSocket);
+      }),
+    );
     final Uri webSocketUrl = url.replace(scheme: 'ws').resolve(path);
     final Uri hostUrl = url
         .resolve('host/index.html')

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -674,7 +674,11 @@ class FlutterWebPlatform extends PlatformPlugin {
     }
 
     final Completer<WebSocketChannel> completer = Completer<WebSocketChannel>.sync();
-    final String path = _webSocketHandler.create(webSocketHandler(completer.complete));
+    final String path = _webSocketHandler.create(
+      webSocketHandler((WebSocketChannel webSocket, _) {
+        completer.complete(webSocket);
+      }),
+    );
     final Uri webSocketUrl = url.replace(scheme: 'ws').resolve(path);
     final Uri hostUrl = url
         .resolve('static/index.html')