#36Implement dead-store eliminations

Repro:

let print = @js("console").log;

var z: int = 1 + 1;
z = 1;
print(z);

Becomes:

// Language Compiler v0.1.3. Alpha quality software.
let v0 = 2;
v0 = 1;
console.log(v0);

Instead of

// Language Compiler v0.1.3. Alpha quality software.
console.log(1);

Milestone: later.