## The basics
- [x] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)
## The details
### Resolves
Fixes https://github.com/google/blockly-keyboard-experimentation/issues/515
### Proposed Changes
This adds `canBeFocused()` checks to all the places that could currently cause problems if a node were to return `false`.
### Reason for Changes
This can't introduce a problem in current Core and, in fact, most of these classes can never return `false` even through subclasses. However, this adds better robustness and fixes the underlying issue by ensuring that `getFocusableElement()` isn't called for a node that has indicated it cannot be focused.
### Test Coverage
I've manually tested this through the keyboard navigation plugin. However, there are clearly additional tests that would be nice to add both for the traverser and for `WorkspaceSvg`, both likely as part of resolving #8915.
### Documentation
No new documentation changes should be needed here.
### Additional Information
This is fixing theoretical issues in Core, but a real issue tracked by the keyboard navigation plugin repository.
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