[dart2js] Refine types based on test with late sentinel

A small number of getters for `late final` instance fields with
initializers have better code.

When the initializing expression provably does not write to the backing
field*, the field value is re-used rather than re-loaded for the 'final'
check (that the field has not been assigned during the evaluation of the
initializer). We are testing both `value === $` and `value !== $`:

```
      value = _this.___LayoutWidgetState_firstChild_FI;
      if (value === $) {
        result = A._LayoutWidgetState__buildChild(B.ValueKey_1, _this._widget.node.firstChild);
        value !== $ && A.throwUnnamedLateFieldADI();
        _this.___LayoutWidgetState_firstChild_FI = result;
        value = result;
      }
```

In this change we refine the type of the tested value to late sentinel,
allowing the check to be removed:

```
      if (value === $)
        value = _this.___LayoutWidgetState_firstChild_FI = A._LayoutWidgetState__buildChild(B.ValueKey_1, _this._widget.node.firstChild);
```

*This effect analysis is pretty simple ('writes some field'), but if
improved, we might expect more `late final` getters to benefit.

Change-Id: I444ed157083663b848fb2f115d0575e544b47727
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/430962
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

https://dart.googlesource.com/sdk/+/e2fdf18b66ba7da09f60f9d91966a86d7e38f827