This fixes an incorrect usage of String::Flatten in EscapeRegExpSource.

It also adds %ConstructConsString (to easily and reliably construct cons
strings in tests) and Factory::NewConsString (to enable guaranteed cons
string construction without preemptive flattening attempts).

BUG=chromium:698790

Review-Url: https://codereview.chromium.org/2736383003
Cr-Commit-Position: refs/heads/master@{#43686}