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**Do you want to request a *feature* or report a *bug*?**
Reporting a bug.

**What is the current behavior?**
With the recent change to `shouldConstruct` (https://github.com/facebook/react/commit/5031ebf6beddf88cac15f4d2c9e91f8dbb91d59d#diff-9ba7cab7f838d9f9071d2fb8ccbf48bfL282) the logic for checking the `isReactComponent` prototype property has become more strict. It turns out we are using a few dependencies that are defining a method called `isReactComponent` and this update caused these components to be treated as an `IndeterminateComponent` type internally.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:**
https://codesandbox.io/s/v3qw39wky

**What is the expected behavior?**
It would be nice if React could recognize this case and maybe log an error in development. I'm happy to try implementing this if you want to go that route. People definitely shouldn't be defining `isReactComponent` on their own components, but if it happens a quick check and an error message would make it a quick fix.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Impacts only v16.5.0. It wasn't an issue due to the less exhaustive check in earlier versions.

static contextTypes = {
        flag: PropTypes.bool,
      };
      render() {
        return <div />;
      }
    }
    class Wrapper extends React.Component {
      render() {
        return (
          <Parent ref="parent">
            <Child ref="child" />
          </Parent>
        );
      }
    }
    const wrapper = ReactTestUtils.renderIntoDocument(<Wrapper />);
    expect(wrapper.refs.parent.state.flag).toEqual(true);
    expect(wrapper.refs.child.context).toEqual({flag: true});
    
    wrapper.refs.parent.setState({flag: false});
    expect(wrapper.refs.parent.state.flag).toEqual(false);
    expect(wrapper.refs.child.context).toEqual({flag: false});
  });
  it('should pass context transitively', () => {
    let childInstance = null;
    let grandchildInstance = null;
    class Parent extends React.Component {