Eliminate redundant instanceof check in JS output

[JakeWharton]

Before:

protoOf(IntAndLong).equals = function (other) {
  if (this === other)
    return true;
  if (!(other instanceof IntAndLong))
    return false;
  var tmp0_other_with_cast = other instanceof IntAndLong ? other : THROW_CCE();
  if (!(this.v_1 === tmp0_other_with_cast.v_1))
    return false;
  if (!this.w_1.equals(tmp0_other_with_cast.w_1))
    return false;
  return true;
};

After:

protoOf(IntAndLong).equals = function (other) {
  if (this === other)
    return true;
  if (!(other instanceof IntAndLong))
    return false;
  if (!(this.v_1 === other.v_1))
    return false;
  if (!this.w_1.equals(other.w_1))
    return false;
  return true;
};

Refs #204