Description

Fix from last time: ensure that each separate artifact has a distinct stamp file, otherwise the order in which artifacts are downloaded will break us.

Attempts to make artifact down-loading lazy. Classifies each CachedArtifact as being either universal or supporting a specific platform. Refactors the EngineArtifact class into a universal, android, or iOS category.

Each command must identify which artifacts it requires statically. Currently all commands required universal, android, and iOS. The web sdk is now only downloaded when running flutter build web

flutter precache now accepts additional flags to force downloading of the artifacts, the defaults include android, ios, and universal.

To improve downloading further, we need to identify which artifacts are required in the build commands
based on the target platform.

Related Issues

Work towards #6491

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