generate-theme command puts quotes around YAML keys with spaces in them

Problem/Motivation

The generate-theme command puts quotes around the 'base theme' YAML key in the generated theme's info.yml file, because it has spaces in it:

'base theme': stable9

Steps to reproduce

Do

$ php core/scripts/drupal generate-theme my_theme

and inspect the generated code.

Proposed resolution

AFAIK from my work on Module Builder, this is caused by Symfony's YAML dumper component.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet