Mojo has a font service, which takes a URL and hands back glyphs.
Sky has an API that takes a URL, a name, and hands back a promise which, when resolved, indicates that Sky has now associated that font name with the relevant glyphs (by calling the Mojo service).
The text part of Sky's drawing API accepts a list of font names, and uses those to draw the relevant glyphs, falling back through the provided fonts, then all the loaded fonts, until a glyph is found.
By default, three fonts are loaded, with the names ‘serif’, ‘sans-serif’, and ‘monospace’. They have good Unicode coverage.
TODO(ianh): Actually define these APIs and so on.