download fonts concurrently with wasm

[alanwutang11]

refactored FontCollection for both canvaskit and html.

With the refactor, it becomes possible to download fonts concurrently with wasm, resolving issue: flutter/flutter#105929

fontCollection refactor

Each font needs to be 1) downloaded and 2) registered before it can be used.

Previously, this process was done in one function call: registerFonts()

Now, downloading fonts happens in two separate function calls:

1. downloadAsetFonts(AssetManager assetmanager) to download fonts
2. registerDownloadedFonts() to register fonts

Breaking the previous registerFonts() into the two aforementioned steps allows the following functions to be called concurrently:

1. download fonts with downloadAsetFonts(AssetManager assetmanager)
2. download wasm with canvaskit = await downloadCanvasKit()

All affected tests were refactored as well.

Before:

After:

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[harryterkelsen]

You should check that the fonts are downloaded early when using the HTML renderer as well (--web-renderer=html).

I think the FontCollection interface is overdue for a refactoring since the names of the methods don't match what is actually happening.

The new FontCollection should contain:

1. loadFontFromList: this is required since Flutter gives developers an API to load a new font. This needs to be Future<void> because the HTML font collection uses an API which is Future
2. downloadAssetFonts: this should be Future<void> and complete when all of the fonts which are read from the FontManifest.json have been downloaded.
3. debugDownloadTestFonts: does what debugRegisterTestFonts does now.
4. registerDownloadedFonts: In this step you register all of the fonts which were downloaded in downloadAssetFonts with the renderer (in the CanvasKit case, this is where you make the SkTypeface and register it with the TypefaceFontProvider. I don't think this needs to be Future<void>, it could probably just be void.

During initialization you would do Future.wait([initializeRenderer, fontCollection.downloadAssetFonts]) and then after that call registerDownloadedFonts.

[harryterkelsen]

Needs more work to fix polyfill version

[skia-gold]

Gold has detected about 2 new digest(s) on patchset 4.
View them at https://flutter-engine-gold.skia.org/cl/github/36813

[skia-gold]

Gold has detected about 2 new digest(s) on patchset 5.
View them at https://flutter-engine-gold.skia.org/cl/github/36813

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Changes reported for pull request #36813 at sha b2b1a4e

[harryterkelsen]

Mostly LGTM just one more change

[harryterkelsen]

LGTM