fix set diff and loop twice (#8699)

Author: AGawrys

packages/core/utils/src/collection.js

@@ -41,6 +41,11 @@ export function setDifference<T>(a: Set<T>, b: Set<T>): Set<T> {
       difference.add(e);
     }
   }
+  for (let d of b) {
+    if (!a.has(d)) {
+      difference.add(d);
+    }
+  }
   return difference;
 }
 

packages/core/utils/test/collection.test.js

@@ -1,7 +1,11 @@
 // @flow
 
 import assert from 'assert';
-import {objectSortedEntries, objectSortedEntriesDeep} from '../src/collection';
+import {
+  objectSortedEntries,
+  objectSortedEntriesDeep,
+  setDifference,
+} from '../src/collection';
 
 describe('objectSortedEntries', () => {
   it('returns a sorted list of key/value tuples', () => {
@@ -38,3 +42,11 @@ describe('objectSortedEntriesDeep', () => {
     );
   });
 });
+describe('setDifference', () => {
+  it('returns a setDifference of two sets of T type', () => {
+    assert.deepEqual(
+      setDifference(new Set([1, 2, 3]), new Set([3, 4, 5])),
+      new Set([1, 2, 4, 5]),
+    );
+  });
+});