https://codereview.chromium.org/2694003002/ introduced
"SyntaxError: Lexical declaration cannot appear in a single-statement context"
for the case when let + desctructuring from a list happen.

As was pointed out in https://codereview.chromium.org/2694003002/#msg18, the
case without destructuring would also benefit from a better message: if a
single statement is expected and "let identifier = ..." is seen, the error is
indeed again that the lexical declaration is not a statement. However, the current
error is "Unexpected identifier", because the parser tries to accept "let" as
an identifier in an expression statement, and then gives up seeing the other
identifier after "let".

This CL ensures that the parser recognises the error properly and reports
accordingly. It also renames the existing test, which contains destructuring,
and adds the one with a non-destructuring lexical declaration.

BUG=v8:5686

Review-Url: https://codereview.chromium.org/2697193007
Cr-Commit-Position: refs/heads/master@{#43275}