The `sdkRoot` argument to start is not honoured everywhere

Notice how in `lib/src/frontend_server_client.dart`:
```dart
final _dartAotRuntimePath = p.join(sdkDir, 'bin', 'dartaotruntime');

final _feServerAppJitSnapshotPath =
p.join(sdkDir, 'bin', 'snapshots', 'frontend_server.dart.snapshot');

final _feServerAotSnapshotPath =
p.join(sdkDir, 'bin', 'snapshots', 'frontend_server_aot.dart.snapshot');
```

just use `sdkDir` which is set in `lib/src/shared.dart`
```dart
final sdkDir = p.dirname(p.dirname(Platform.resolvedExecutable));
```
Problem is that `Platform.resolvedExecutable` doesn't work with `dart install`.

However
```dart
static Future start(
...
String? sdkRoot, // Defaults to the current SDK root.
})
```
if only it was used correctly, like:
```dart
String _dartAotRuntimePath(String? sdkRoot) =>
p.join(sdkRoot ?? sdkDir, 'bin', 'dartaotruntime');

String _feServerAppJitSnapshotPath(String? sdkRoot) => p.join(
sdkRoot ?? sdkDir, 'bin', 'snapshots', 'frontend_server.dart.snapshot');

String _feServerAotSnapshotPath(String? sdkRoot) => p.join(
sdkRoot ?? sdkDir, 'bin', 'snapshots', 'frontend_server_aot.dart.snapshot');
```
then it would work with `dart install` as well, if we pass the `sdkRoot` argument.

Start in lib/src/frontend_server_client.dart and inspect FrontendServerClient.start, then compare its sdkRoot handling with sdkDir defined in lib/src/shared.dart. Trace the three frontend-server runtime and snapshot paths and verify that an explicitly supplied SDK root is used, including when invoked through dart install.

Written by the indexing model from the issue text.