Fine grained selection of 'late' lowering by front end.

[rakudrama]

dart2js would like to lower some late variables and leave others to the front-end.

The number of configurations is large (3×2×2×2 = 24):

{static, instance, local} × {final, non-final} × {initializer, uninitialized} × {nullable, non-nullable}

Where possible, dart2js would like to be able to inspect the initializer and variable type.

[rakudrama]

/cc @johnniwinther

[johnniwinther]

We could also make the flag in Target a bit mask of requested lowerings:

class Target {
  ...
  int get lateLowerings;
  ...
}

class LateLowering {
  static const int nullableUninitializedNonFinalLocal = 1 << 0;
  static const int nonNullableUninitializedNonFinalLocal = 1 << 1;
  static const int nullableUninitializedFinalLocal = 1 << 2;
  static const int nonNullableUninitializedFinalLocal = 1 << 3;
  static const int nullableInitializedNonFinalLocal = 1 << 4;
  static const int nonNullableInitializedNonFinalLocal = 1 << 5;
  static const int nullableInitializedFinalLocal = 1 << 6;
  static const int nonNullableInitializedFinalLocal = 1 << 7;
  static const int uninitializedNonFinalStaticField = 1 << 8;
  static const int uninitializedFinalStaticField = 1 << 9;
  static const int initializedNonFinalStaticField = 1 << 10;
  static const int initializedFinalStaticField = 1 << 11;
  static const int uninitializedNonFinalInstanceField = 1 << 12;
  static const int uninitializedFinalInstanceField = 1 << 13;
  static const int initializedNonFinalInstanceField = 1 << 14;
  static const int initializedFinalInstanceField = 1 << 15;

  static const int all = (1 << 16) - 1;
}

Here the variants with nullable/non-nullable instance/static fields are excluded. The type of fields rely on top level inference but we set up for lowering of fields when we see their declaration.

This approach would also decouple the decision from the inner workings of the CFE, i.e. knowing how to correctly detect whether a type is nullable or not, or how to detect if an initializer is declared or synthesized.