blocks: save/download and re-load doesn't preserve blocks program

If a user is coding using blocks, the blockly content gets lost at the 'save', 'download' or 'share' stage.

This makes it hard to, for example, share skeleton blocks programs, or even return to a previously incomplete blocks program.

No files or tests are named. Reproduce the issue by creating a blocks program, then using save, download, and share before re-loading it; trace each persistence path and compare the restored editor state. Done means blocks content survives all three flows and an incomplete program can be reopened.

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