Restrict the use of the `await` token as an identifier when parsing
source text as module code.

From
http://www.ecma-international.org/ecma-262/6.0/#sec-future-reserved-words:

> 11.6.2.2 Future Reserved Words
>
> The following tokens are reserved for used as keywords in future
> language extensions.
>
> Syntax
>
>     FutureReservedWord ::
>         enum
>         await
>
> await is only treated as a FutureReservedWord when Module is the goal
> symbol of the syntactic grammar.

BUG=v8:4767
LOG=N
R=adamk@chromium.org

Review-Url: https://codereview.chromium.org/1723313002
Cr-Commit-Position: refs/heads/master@{#35914}