Repro for "expect trees to be >=2 elements long" invariant

These cases have a switch with only a default, so we create a "conditional" 
child dependency tree and no other trees which hits the invariant.

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-switch-with-only-default.expect.md

@@ -0,0 +1,21 @@
+
+## Input
+
+```javascript
+function Component({ kind, ...props }) {
+  switch (kind) {
+    default:
+      return <Stringify {...props} />;
+  }
+}
+
+```
+
+
+## Error
+
+```
+[ReactForget] Invariant: Expected trees to be at least 2 elements long.
+```
+          
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-switch-with-only-default.js

@@ -0,0 +1,6 @@
+function Component({ kind, ...props }) {
+  switch (kind) {
+    default:
+      return <Stringify {...props} />;
+  }
+}