Don't generate const variables as lazy getters

[leafpetersen]

Not high priority, but something to consider.

I see code that looks like:

const int TAG_TYPE_BITS = 3;

Generating code that looks like:

dart.defineLazy(protobuf, {
  get TAG_TYPE_BITS() { 
    return 3;
  },

Would probably be worth just generating direct properties for these kind of things.

[jmesserly]

Hmmm. We used to optimize more of these, I'm surprised we don't do that here. I think it gets tricky if we're dealing with user classes and ordering issues though, so that may be why we backed off. I did think JSON stuff was still optimized though.

[jmesserly]

I also think we could do better for lazy getters in general.