[TypeScript][Angular2] Add back missing changes from #4539 due to bad merge

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

For #4539 (comment)

[wing328]

@kernwig I only found 1 line of change missing from #4539. I wonder if you can review to see if I'm missing anything. Thanks.