Currently, dynamic `import()` throws the following SyntaxError when
used without a specifier:

    > import();
    < Uncaught SyntaxError: Unexpected token )

From the error message, it seems this the result of the code snippet
being seen as static `import` followed by parens, as opposed to
`import()` with no specifier.

This patch makes this error message more clear:

    > import();
    < SyntaxError: import() requires a specifier

BUG=v8:7020,v8:6513

Change-Id: I3519dfd0029f38d23da858a5499f1d226e794935
Reviewed-on: https://chromium-review.googlesource.com/747141Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49058}