Fixes https://github.com/flutter/flutter/issues/136698.

Alters how `throwToolExit` creates its matcher. This results is an improved description of the matcher.

The mismatch description isn't improved by this, but I writing an entirely custom matcher to fix this isn't ideal either. We can instead mitigate the issue by augmenting the `toString` implementation of `ToolExit` to include the exit code, if it is non-null.

With these changes, the first few lines of output from a test would look like this:

```
Expected: throws <Instance of 'ToolExit'> with `exitCode`: <42> and `message`: contains 'message'
  Actual: <Closure: () => Never>
   Which: threw ToolExit:<Exit code: 41232. Error: message>
```