Using a temporary flag to toggle the default font. This is needed forflutter/engine#40352, and the changes will be removed once that PR lands in flutter/flutter.

Pre-launch Checklist

• I read theContributor Guide and followed the process outlined there for submitting PRs.
• I read theTree Hygiene wiki page, which explains my responsibilities.
• I read theFlutter Style Guiderecently, and have followed its advice.
• I signed theCLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with///).
• I added new tests to check the change I am making, or this PR istest-exempt.
• All existing and new tests are passing.

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