[fixkeystypes] Improve keys typing (#321)

## Summary:
I found that the typing was not helpful so I added a test case for the issue I experienced and then tweaked the function definition so that all the flow tests passed.

No functional change here, purely types.

Issue: FEI-4655

## Test plan:
`yarn flow`

Author: somewhatabstract

Reviewers: jeresig, benchristel, somewhatabstract

Required Reviewers:

Approved By: jeresig

Checks: ✅ Test (macOS-latest, 16.x), ✅ codecov/project, ✅ CodeQL, ✅ Lint, flow, and coverage check (ubuntu-latest, 16.x), ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 16.x), ✅ gerald, ✅ Analyze (javascript), ⏭  dependabot

Pull Request URL: #321

.changeset/polite-snails-grab.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-stuff-core": patch
+---
+
+Update types for `keys` so that it works with a wider variety of object shapes

packages/wonder-stuff-core/src/__tests__/keys.flowtest.js

@@ -32,3 +32,14 @@ import {keys} from "../keys.js";
     const keys2bad = keys(obj2);
     const __: "a" = keys2bad[0];
 }
+
+{
+    // should work with more specific object types
+    const obj3: {|[string]: string|} = {
+        a: "1",
+        b: "2",
+    };
+
+    // This should not be erroring.
+    const _ = keys(obj3);
+}

packages/wonder-stuff-core/src/keys.js

@@ -6,6 +6,6 @@
  * to be returned.
  * @returns {Array<$Keys<O>>} An array of the enumerable keys of an object.
  */
-export function keys<O: {[string]: mixed}>(obj: O): Array<$Keys<O>> {
+export function keys<O: {[string]: mixed}>(obj: $ReadOnly<O>): Array<$Keys<O>> {
     return Object.keys(obj);
 }