Revert "Prevent solo: true from being committed" (#51858)

Reverts #51712

Reason for revert: As discussed in
flutter/flutter#143312, and in
https://discord.com/channels/608014603317936148/1224499330824802375. I
apologize if this seems hasty and not fully justified, but it is
important that the engine repo follows policies like this to make it
easier for our various teams to work together.

Author: zanderso

lib/web_ui/analysis_options.yaml

@@ -6,12 +6,6 @@
 
 include: ../../analysis_options.yaml
 
-analyzer:
-  errors:
-    # We need this in the web engine in order to prevent committing `solo: true`
-    # in tests.
-    deprecated_member_use: true
-
 linter:
   rules:
     avoid_dynamic_calls: false

lib/web_ui/dev/test_platform.dart

@@ -22,7 +22,6 @@ import 'package:shelf_web_socket/shelf_web_socket.dart';
 import 'package:skia_gold_client/skia_gold_client.dart';
 import 'package:stream_channel/stream_channel.dart';
 
-// ignore: deprecated_member_use
 import 'package:test_core/backend.dart' hide Compiler;
 // TODO(ditman): Fix ignores when https://github.com/flutter/flutter/issues/143599 is resolved.
 import 'package:test_core/src/runner/environment.dart'; // ignore: implementation_imports