code: function(keyDownFn, keyUpFn) {
      var g_keyState = {};
      var g_oldKeyState = {};
  
      var updateKey = function(keyCode, state) {
        g_keyState[keyCode] = state;
        if (g_oldKeyState !== g_keyState) {
          g_oldKeyState = state;
          if (state) {
            keyDownFn(keyCode);
          } else {
            keyUpFn(keyCode);
          }
        }
      };
  
      var keyUp = function(event) {
        updateKey(event.keyCode, false);
      };
  
      var keyDown = function(event) {
        updateKey(event.keyCode, true);
      };
  
      window.addEventListener("keyup", keyUp, false);
      window.addEventListener("keydown", keyDown, false);
    }
docs: Sets up controller key functions
  @param {callback(code, down)} keyDownFn a function to be
  called when a key is pressed. It's passed the keycode
  and true.
  @param {callback(code, down)} keyUpFn a function to be called
  when a key is released. It's passed the keycode and
  false.
  @memberOf module:Input