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.*

- dart-lang/shelf#457
- dart-lang/shelf#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.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
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[CLA]: https://cla.developers.google.com/
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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

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')