transformer: object rest spread remaining issues

[Boshen]

• suboptimal output for ({ a2, ...b2 } = c2)

var _c;
_c = c2, {a2} = _c, b2 = babelHelpers.objectWithoutProperties(_c, ["a2"]), _c;
                                                 // This should be removed ^^

• incorrect transform for ({ ...x }, y = x) => { }

(_ref, y = x) => {
        // ^ Need to handle this assignment
	let x = babelHelpers.extends({}, (babelHelpers.objectDestructuringEmpty(_ref), _ref));
};

• compiler assumption objectRestNoSymbols
• compiler assumption ignoreFunctionLength
• option loose
• useBuiltIns