ASI incorrectly kicks in for let declarations

[DanielRosenwasser]

From looking through the specification, I don't think we are doing the right thing when a let declaration spans multiple lines.

The following:

let
x

seems to incorrectly compile to

let;
x;

instead of

var x;

I'm not seeing a "[No LineTerminator here]" for let declarations in the spec. The most relevant restriction on let that I see is the following:

13.4 Expression Statement

Syntax

ExpressionStatement_[Yield] :

      [lookahead ∉ {{, function, class, let [}] Expression_{[In,\ ?Yield]} ;

NOTE [...] An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

[CyrusNajmabadi]

This is because it would otherwise be a breaking change moving from ES5 to ES6. In ES5:

let
x

Is parsed as

let;
x;

So we need to preserve that. In ES5 though let x won't legally parse, so it's ok to now parse that as a let-declaration.

[CyrusNajmabadi]

Interesting point about let [. It would seem like they're allowing a breaking change there. someone could have had "let[0] = 1" in their existing ES5 code, and that would break moving to ES6.

[DanielRosenwasser]

Discussed offline with @CyrusNajmabadi; while the above was our reason for implementing things the way they are now, we're leaning towards just aligning with the ES6.