Before this change, load loads fonts in a non-deterministic order. After this change, it loads the fonts sequentially.

See cl/797583429 for the full context. We found that the order of how fonts load for a specific font family actually affects runtime behavior, and the current non-deterministic order of loading fonts leads to flakiness.

Consider the following:

• foo.ttf has glyphs 'A' and 'B'
• foo-italic.ttf only has glyph 'B'

If addFont(foo-italic.ttf) is called before addFont(foo.ttf), the font loader will load foo-italic.ttf first and use that as the base font. When looking for the 'A' glyph, it may will look for the glyph only in foo-italic.ttf, which doesn't have it, and will show an incorrect empty placeholder glyph.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• 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 is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.