Allow referring to assigned variable within its initializer.

Cascade syntax helps avoid redundant references to the same variable, but it breaks down once you need to reference that variable within a cascade call.

The following are logically equivalent:
```dart
var foo = bar();
foo.baz();
```

```dart
var foo = bar()
..baz();
```

However the same equivalence can't be made with:

```dart
var foo = bar();
foo.baz(() {
foo.blub();
});
```

```dart
var foo = bar()
..baz(() {
foo.blub(); // Error, "Local variable 'foo' can't be referenced before it is declared"
});
```

Start with the issue's cascade and initializer examples and read the discussion for the intended language semantics. Determine whether closures inside an initializer may refer to the variable being declared, including the required diagnostic or specification behavior; done means the proposal has an agreed, precise resolution.

Written by the indexing model from the issue text.