Undo broken when deleting multiple blocks in a stack

[RoboErikG]

Problem statement

If you delete a stack of blocks (via delete all for instance) the undo stack ends up in a bad state.

Expected Behavior

Can delete a stack of blocks and then undo it.

Actual Behavior

Can't undo the deletion correctly, just get the first block.

Steps to Reproduce

1. On develop version of playground, create a stack of blocks
2. delete all blocks through the right click meno.
3. Undo

Stack Traces

block_events.js:286 Can't uncreate non-existent block: wmiK%..C4a9ME{sFJN?m
Blockly.Events.Create.run @ block_events.js:286

Replace with error stack trace.

Operating System and Browser

Additional Information

[RoboErikG]

Same root cause as #2096