Use block-scoped variables in `while`

Currently this does not work:

```dart
void main() {
do {
int n = 1;
} while (n > 1); // Error: Undefined name 'n'.
}
```

This is because the scope of `n` ends with `}`

However, this does not mirror the way that `for` works. `for` creates a scope that end with `}` despite that the variable is defined outside of `{}`:

```dart
void main() {
for (int n in [1]) {}
n = 2; // Error: Setter not found: 'n'.
}
```

If this suggestion is implemented, conditional loops can be made easier.

The issue names no file, test, or entry point. Start by comparing the specified scoping rules for do-while and for loops; done means the requested behavior is resolved consistently with the examples.

Written by the indexing model from the issue text.