This commit adds a fallback when attempting to merge events that
collide on a hash but do not actually merge. Previously, the latter
event would be dropped.
This also fixes a minor UX problem where moving the isNull check into
the merging loop resulted in extra events in the undo queue that
served no purpose and required multiple undo operations without visual
feedback.
Fixes#1373
* Implement Blockly.Events.filter in linear time
For large App Inventor projects (order 1k+ blocks, 100+ top-level
blocks), the O(n^2) behavior of Blockly.Event.filter was causing
performance issues when rearranging blocks or pasting from the
backpack. This commit provides a linear merge implementation using a
key that uniquely identifies a block so that multiple events targeting
the same block are merged. This change benefits from O(1) amortized
lookup using an object as a key-value store.
* Add event filter unit tests and fix logic bugs
* Update Blockly.Events.filter unit tests
