Move 'throw' to Expression

[rbuckton]

This is one of two alternative approaches to define throw expressions such that their operand has the same precedence as the operand to a ThrowStatement. For the alternative approach, see #22.

In this variant, ThrowExpression is a direct descendant of Expression, but is mutually exclusive with the , operator. This means that ThrowExpression will almost always require parentheses, though it is arguable as to whether the few places where parentheses can be elided are useful:

// moderately useful when paired with `break`, allowed without needing extra parenthesis:
do { } while (throw a); 
for (; throw a;) {}
for (;; throw b) {}

// nonsensical, but allowed without needing extra parentheses:
if (throw a) { }
while (throw a) { }
for (throw a;;) { }
for (let x in throw a) { }
switch (throw a) { }
switch (a) { case throw b: }
return throw a;
throw throw a;
`${throw a}`;
a[throw b];
a?.[throw b];

Fixes #23

[github-actions]

A preview of this PR can be found at https://tc39.es/proposal-throw-expressions/pr/21.

[rbuckton]

@bakkot, @michaelficarra, @nicolo-ribaudo: could you take a look?

[michaelficarra]

Doesn't this introduce an ambiguous parse between throw statements and throw expressions in statement position?

edit: I didn't notice the lookahead since it wasn't part of the diff. Shouldn't be an issue, then.

[nicolo-ribaudo]

I very much prefer #22 as it makes the rule for parentheses easier, but the spec text here looks good if we were to choose this syntax.