Follow up from https://github.com/facebook/react/pull/11616.

Here's one I found. These two invariants:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L330-L343

are in a function that's behind a DEV block:

https://github.com/facebook/react/blob/7e7127387ba64f57299eaff8656542cea6fb3a2d/packages/react-reconciler/src/ReactFiberClassComponent.js#L447-L449

I don't think this is intentional, but need to verify.

There may be more. I'll use this issue to track while https://github.com/facebook/react/pull/11616 preserves existing behavior.
itRenders('download prop with number 0 value', async render => {
        const e = await render(<a download={0} />);
        expect(e.getAttribute('download')).toBe('0');
      });
      itRenders('no download prop with null value', async render => {
        const e = await render(<div download={null} />);
        expect(e.hasAttribute('download')).toBe(false);
      });
      itRenders('no download prop with undefined value', async render => {
        const e = await render(<div download={undefined} />);
        expect(e.hasAttribute('download')).toBe(false);
      });
    });
    describe('className property', function() {
      itRenders('className prop with string value', async render => {
        const e = await render(<div className="myClassName" />);
        expect(e.getAttribute('class')).toBe('myClassName');
      });
      itRenders('className prop with empty string value', async render => {
        const e = await render(<div className="" />);
        expect(e.getAttribute('class')).toBe('');
      });
      itRenders('no className prop with true value', async render => {
        const e = await render(<div className={true} />, 1);
        expect(e.hasAttribute('class')).toBe(false);
      });
      itRenders('no className prop with false value', async render => {
        const e = await render(<div className={false} />, 1);
        expect(e.hasAttribute('class')).toBe(false);
      });