The previous code had a mix of breaks, early returns, and switch/case/if
with fallthrough. Now the pattern is to either return for known errors
or break to the bottom of the switch for unhandled tokens.

Also cleaned up random other stuff in the function: removed unnecessary
local vars, shortened position-fetching calls.

Review URL: https://codereview.chromium.org/1412313009

Cr-Commit-Position: refs/heads/master@{#31843}