support const in frog

[sigmundch]

Frog currently correctly makes 2 const expressions have the same runtime value if:

• they are identical expressions syntactically (const A(1) == const A(1)), or
• they are identical after folding arithmetic operations, and static constants (const A (1 + 1) == const A(2))

We need to extend this to support equivalence of instances built with different constructors, for instance the following should be true:
const B() == const B(2) == const B(2, 0)

where,
class A {
  final int x;
  const A(this.x);
}

class B extends A {
  final int y;
  const B(): this(2);
  const B(x, this.y): super(x);
  const B(x): y = 0, super(x);
}

[sigmundch]

Added Fixed label.