IIFE tracking issue.

This issue keeps track of several issues related to immediately invoked function expressions ([IIFE](https://en.wikipedia.org/wiki/Immediately_invoked_function_expression)s).

### Issues that prevent IIFEs from acting like an expression that contains a list of statements:
* https://github.com/dart-lang/language/issues/2848
* https://github.com/dart-lang/language/issues/2820

### Language tour:
* https://github.com/dart-lang/site-www/issues/4613

### Making IIFEs amenable to custom analyses:
* https://github.com/dart-lang/language/issues/2852

### Refactoring support:
* https://github.com/dart-lang/sdk/issues/51423
* https://github.com/dart-lang/sdk/issues/51421

### Performance
* https://github.com/dart-lang/sdk/issues/52694

This is a tracking issue rather than a self-contained task. Review the linked Dart language issues 2848, 2820, and 2852, the language-tour issue 4613, and the SDK issues 51423, 51421, and 52694 to understand the separate workstreams. Done means resolving or coordinating those linked issues, not changing a file named here.

Written by the indexing model from the issue text.