IT WORKS!! (sort of... it breaks `arguments` slightly and doesn't work when optimized :( --- oh and doesn't work at all for arrow functions .____.)

results.sh

[00:00|% 100|+   0|-   1]: Done                       
EchoBeach-2:v8 caitp$ out/x64.release/d8 --harmony-rest-parameters test.js
STRICT MODE
[]
[2,3]
[2]


SLOPPY 'FAST' MODE
[]
[2,3]
[2]


SLOPPY 'SLOW' MODE
[1]
[1,2,3]
[1,2]

test.js

(function() {
  "use strict";

  function fn(a, ...b) {
    return b;
  }

  print("STRICT MODE");
  print(JSON.stringify(fn(1)));
  print(JSON.stringify(fn(1, 2, 3)));
  print(JSON.stringify(fn(1, 2)));
  print("\n");
})();

(function() {
  function fn(a, ...b) {
    return b;
  }

  print("SLOPPY 'FAST' MODE");
  print(JSON.stringify(fn(1)));
  print(JSON.stringify(fn(1, 2, 3)));
  print(JSON.stringify(fn(1, 2)));
  print("\n");
})();

(function() {
  function fn(a, a, ...b) {
    return b;
  }

  print("SLOPPY 'SLOW' MODE");
  print(JSON.stringify(fn(1, 1)));
  print(JSON.stringify(fn(1, 1, 2, 3)));
  print(JSON.stringify(fn(1, 1, 2)));
  print("\n");
})();