It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
render() {
          return <div>Child2</div>;
        }
      }
      class Child3 extends React.Component {
        render() {
          return <div onclick="1">Child3</div>;
        }
      }
      class Child4 extends React.Component {
        render() {
          return <div>Child4</div>;
        }
      }
      ReactTestUtils.renderIntoDocument(<Parent />, container);
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('className');
      var matches = console.error.calls.argsFor(0)[0].match(/.*\(.*:(\d+)\).*/);
      var previousLine = (matches || [])[1];
      expectDev(console.error.calls.argsFor(1)[0]).toContain('onClick');
      matches = console.error.calls.argsFor(1)[0].match(/.*\(.*:(\d+)\).*/);
      var currentLine = (matches || [])[1];
      
      
      expect(parseInt(previousLine, 10) + 12).toBe(parseInt(currentLine, 10));
    });
    it('gives source code refs for unknown prop warning for exact elements in composition (ssr)', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      class Parent extends React.Component {