fix(tests): revert change in marker_svg to keep keyboard nav tests working

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Resolves one of the issues blocking google/blockly-samples#1297 (comment)

Proposed Changes

Use a cast and checking for a property, rather than an instanceof check, to make it safe to call beginElement() on a marker. This avoids breaking tests when run in jsdom.

Behavior Before Change

Tests broke becuase SVGAnimationElement was not defined when running with jsdom.

Behavior After Change

These tests pass (although a different set of tests fails >.<)

Reason for Changes

The tests are running in Mocha, and using jsdom to pretend that they are in a rendered page. The marker SVG and similar elements are created but certain classes don't exist. SVGAnimationElement is one of them, which triggered a failure.

Note that the tests running rendered in jsdom is a bit weird, but they cannot run in headless mode because keyboard navigation is not built for non-rendered workspaces.

Test Coverage

I built, packaged, and linked changes so that I could run keyboard navigation tests. The tests no longer fail on SVGAnimationElement, although two tests still fail.

Documentation

Additional Information

Once this change is in Beka will cut a beta so that we can test it over on samples and move the blocky-9-beta branch one step closer to working.