Can not use `await` in Dart expression evaluation

When using `await` in a Dart expression, you'll get the following error:
```
CompilationError: 'await' can only be used in 'async' or 'async*' methods.
```

The issue names Dart expression evaluation but provides no file, test, or implementation entry point. Start by locating the expression-evaluation path and its existing async handling, then determine the expected behavior for await expressions and add coverage showing that valid async evaluation completes without the reported compilation error.

Written by the indexing model from the issue text.