Fix race condition in 'fetch-parserTests.txt.js --force'.

We were computing the SHA1 hash of the newly-downloaded file before
node has finished writing it and closing the file, which sometimes
caused the wrong hash to be written.

Change-Id: I51eac688d477af28ee7dc05092141d42520eecde
Merge "Fix race condition in 'fetch-parserTests.txt.js --force'."