For consideration.

These warnings are already covered by renderers based on react-reconciler. Maybe we don't gain that much by mirroring them in the shallow renderer? Maybe it's not worth the added complexity? Particularly with more complicated warnings like the ones being added for #12419 and #12404.
}
    SomeComponent.contextTypes = {
      bar: PropTypes.string,
    };
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent foo={'FOO'} />, {
      bar: 'BAR',
    });
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <div>FOO</div>,
      <div>BAR</div>,
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should shallow render a component returning strings directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value="foo" />);
    expect(result).toEqual('foo');
  });
  it('should shallow render a component returning numbers directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value={10} />);
    expect(result).toEqual(10);
  });
  it('should shallow render a fragment', () => {
    class SomeComponent extends React.Component {