Handle nested Fragments in toTree (facebook#12106)

Author: maciej-ka

packages/react-test-renderer/src/ReactTestRenderer.js

@@ -338,6 +338,8 @@ function toTree(node: ?Fiber) {
       };
     case HostText: // 6
       return node.stateNode.text;
+    case Fragment: // 10
+      return toTree(node.child);
     default:
       invariant(
         false,

packages/react-test-renderer/src/__tests__/ReactTestRenderer-test.js

@@ -530,6 +530,24 @@ describe('ReactTestRenderer', () => {
     );
   });
 
+  it('toTree() handles nested Fragments', () => {
+    const Foo = () => <React.Fragment><React.Fragment>foo</React.Fragment></React.Fragment>;
+    const renderer = ReactTestRenderer.create(<Foo />);
+    const tree = renderer.toTree();
+
+    cleanNodeOrArray(tree);
+
+    expect(prettyFormat(tree)).toEqual(
+      prettyFormat({
+        nodeType: "component",
+        type: Foo,
+        instance: null,
+        props: {},
+        rendered: "foo",
+      }),
+    );
+  })
+
   it('toTree() handles null rendering components', () => {
     class Foo extends React.Component {
       render() {