infer types of const

[jmesserly]

from the DeltaBlue code:

const WEAKEST          = const Strength(6, "weakest");
// ...
class Variable {
  // ...

  // Generates as:
  // this.walkStrength = /* Unimplemented: DownCast: dynamic to Strength */ WEAKEST;
  Strength walkStrength = WEAKEST;
  // ...
}

that would not be a very useful down cast :)

[sigmundch]

totally I agree - I think this is a dupe of #8 though.