[web] keep_alive_test 'KeepAlive render tree description' fails on web.

[darrenaustin]

Part of the skip test audit. The test:

flutter/packages/flutter/test/widgets/keep_alive_test.dart

Line 193 in d3bdbed

testWidgets('KeepAlive render tree description', (WidgetTester tester) async {

Is currently skipped on the web. If we reenable it we get:

%  flutter test --platform=chrome --web-renderer=html test/widgets/keep_alive_test.dart 
00:03 +3: KeepAlive render tree description                                                                                                                                                                                                                                                                       
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following UnsupportedError was thrown running a test:
Unsupported operation: Platform._operatingSystem

When the exception was thrown, this was the stack:
../dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 251:49     throw_
../dart-sdk/lib/_internal/js_dev_runtime/patch/io_patch.dart 244:5                  _operatingSystem
../dart-sdk/lib/io/platform_impl.dart 56:40                                         get operatingSystem
../dart-sdk/lib/io/platform.dart 64:45                                              get _operatingSystem
../dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 835:8  get
../dart-sdk/lib/io/platform.dart 80:40                                              get operatingSystem
keep_alive_test.dart.js 501:222                                                     <fn>

If this is not a bug, but intentional, just add a comment with an [intended] tag to indicate that this should always be skipped.

flutter doctor

[✓] Flutter (Channel master, 2.5.0-6.0.pre.28, on macOS 11.5.1 20G80 darwin-x64, locale en)
    • Flutter version 2.5.0-6.0.pre.28 at /Users/darrenaustin/dev/flutter
    • Upstream repository git@github.com:flutter/flutter.git
    • Framework revision d3bdbed498 (20 hours ago), 2021-08-06 17:45:02 -0700
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