apollo client:download-schema reports its finished before output file exists

**Intended outcome:**
Running `apollo client:download-schema --endpoint=http://localhost:4000 src/generated/schema.json | cat src/generated/schema.json` should print the json

**Actual outcome:**
cat: src/generated/schema.json: No such file or directory

**Versions**

apollo/2.16.1 win32-x64 node-v10.15.1

Start by running `apollo client:download-schema --endpoint=http://localhost:4000 src/generated/schema.json` with the reported piping command and inspect the CLI entry point for `client:download-schema`. The fix is complete when the command does not report completion until `src/generated/schema.json` exists and the following `cat` prints the JSON.

Written by the indexing model from the issue text.