It originates from the era where we used to run a separate preparse step
before parsing and store the function data. Now the usage of preparser
is something completely different, so this flag doesn't make sense any
more.

In addition, this way we get more test coverage for preparser (for small
scripts).

BUG=

Review-Url: https://codereview.chromium.org/2513563002
Cr-Commit-Position: refs/heads/master@{#41110}