object spread: also drop non-nullish constants (e.g. { ...true })

[WofWca]

This is a follow-up to 6f04338 (#1004)

Note, as I said in #1004, I have not yet found where in the spec it says how spreading constants other than undefined & null should be handled.

[jridgewell]

The spec says:

PropertyDefinition : ... AssignmentExpression

1. Let exprValue be the result of evaluating AssignmentExpression.
2. Let fromValue be ? GetValue(exprValue).
3. Let excludedNames be a new empty List.
4. Return ? CopyDataProperties(object, fromValue, excludedNames).

CopyDataProperties says

7.3.26 CopyDataProperties ( target, source, excludedItems )

The abstract operation CopyDataProperties takes arguments target (an Object), source (an ECMAScript language value), and excludedItems (a List of property keys). It performs the following steps when called:

1. If source is undefined or null, return target.
2. Let from be ! ToObject(source).
3. Let keys be ? from.[[OwnPropertyKeys]]().
4. For each element nextKey of keys, do
   a. Let excluded be false.
   b. For each element e of excludedItems, do
   i. If SameValue(e, nextKey) is true, then
   1. Set excluded to true.
      c. If excluded is false, then
      i. Let desc be ? from.[[GetOwnProperty]](nextKey).
      ii. If desc is not undefined and desc.[[Enumerable]] is true, then
   2. Let propValue be ? Get(from, nextKey).
   3. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
5. Return target.

The important part is:

2. Let from be ! [ToObject](https://tc39.es/ecma262/multipage/abstract-operations.html#sec-toobject)(source).
3. Let keys be ? from.[[OwnPropertyKeys]]().

ToObject of a primitive returns a new object instance (eg, new Number(1), which is a Number instance and not a number primitive). This also applies when spreading a "string", and String instances are the only primitive class which has an own-property, which is what spread operator is iterating over.

[WofWca]

Just realized that it stopped droping AST_RegExp. I think it can still be merged as it's still an improvement overall.

[WofWca]

Alright, I've decided to update it so that it also drops AST_RegExps by explicitly checking if expr instanceof AST_Constant. Maybe is_constant() really could used a rename (#1144 (comment))

[fabiosantoscode]

This looks great, is it good to merge @jridgewell ?

[WofWca]

So, what's the plan for this MR?

[fabiosantoscode]

This looks good. I kind of dropped the ball here, sorry!