[core] Fix more warnings with enzyme@3 and react@16 (#8641)

src/test-utils/until.js

@@ -12,7 +12,8 @@ function shallowRecursively(wrapper, selector, { context, ...other }) {
   // enzyme@3
   // const instance = wrapper.root().instance();
   const instance = wrapper.root.instance();
-  if (instance.getChildContext) {
+  // The instance can be null with a stateless functional component and react >= 16.
+  if (instance && instance.getChildContext) {
     newContext = {
       ...context,
       ...instance.getChildContext(),

src/test-utils/until.spec.js

@@ -80,16 +80,26 @@ describe('until', () => {
     assert.strictEqual(wrapper.contains(<div />), true);
   });
 
-  const Foo = () => <Div />;
-  Foo.contextTypes = { quux: PropTypes.bool.isRequired };
+  // eslint-disable-next-line react/prefer-stateless-function
+  class Foo extends React.Component<{}> {
+    render() {
+      return <Div />;
+    }
+  }
+
+  Foo.contextTypes = {
+    quux: PropTypes.bool.isRequired,
+  };
 
   it('context propagation passes down context from the root component', () => {
     const EnhancedFoo = hoc(Foo);
-    const wrapper = until.call(shallow(<EnhancedFoo />), 'Foo', { context: { quux: true } });
+    const options = { context: { quux: true } };
+    const wrapper = until.call(shallow(<EnhancedFoo />, options), 'Foo', options);
     assert.strictEqual(wrapper.context('quux'), true);
     assert.strictEqual(wrapper.contains(<Div />), true);
   });
 
+  // eslint-disable-next-line react/no-multi-comp
   class Bar extends React.Component<{}> {
     static childContextTypes = { quux: PropTypes.bool };
     getChildContext = () => ({ quux: true });