Prevent solo: true from being committed (#51712)

At some point, we inherited a change from the repo-level `analysis_options.yaml` that [allowed deprecated](#50575) members to be used. We rely on that analyzer error to prevent `solo: true` from being committed in our tests ([example](https://github.com/flutter/engine/blob/b7dddee939f2429e5fd3574a63f9dfac4f486a33/lib/web_ui/test/ui/line_metrics_test.dart#L179) of one that slipped recently).

This PR overrides the `deprecated_member_use` error to enable it inside the web engine.

Author: mdebbar

lib/web_ui/analysis_options.yaml

@@ -6,6 +6,12 @@
 
 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

@@ -22,6 +22,7 @@ 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