Dry layout support for RenderBox

Subclasses that already override performResize can be migrated by simply changing the function signature from void performResize() to Size computeDryLayout(BoxConstraints constraints) and by returning the calculated size instead of assigning it to the size setter. The old implementation of performResize can be removed.

Code before migration:

  @override
  void performResize() {
     size = constraints.biggest;
  }

Code after migration:

  // This replaces the old performResize method.
  @override
  Size computeDryLayout(BoxConstraints constraints) {
     return constraints.biggest;
  }

If the subclass doesn't override performResize, the implementation of computeDryLayout has to be extracted from the performLayout method. Basically, computeDryLayout needs to do all the work performLayout is doing to figure out the size of the RenderBox. However, instead of assigning it to the size setter, it returns the computed size. If computeDryLayout needs to know the size of its children, it must obtain that size by calling getDryLayout on the child instead of calling layout.

If for some reason it is impossible to calculate the dry layout, computeDryLayout must call debugCannotComputeDryLayout from within an assert and return a dummy size of const Size(0, 0). Calculating a dry layout is, for example, impossible if the size of a RenderBox depends on the baseline metrics of its children.

  @override
  Size computeDryLayout(BoxConstraints constraints) {
    assert(debugCannotComputeDryLayout(
      reason: 'Layout requires baseline metrics, which are only available after a full layout.'
    ));
    return const Size(0, 0);
  }