Grabbed dndbeyond's source code

```
~/go/bin/sourcemapper -output ddb -jsurl https://media.dndbeyond.com/character-app/static/js/main.90aa78c5.js
```

ddb_main/node_modules/@mui/utils/composeClasses/composeClasses.js

@@ -0,0 +1,21 @@
+export default function composeClasses(slots, getUtilityClass, classes = undefined) {
+  const output = {};
+  Object.keys(slots).forEach(
+  // `Object.keys(slots)` can't be wider than `T` because we infer `T` from `slots`.
+  // @ts-expect-error https://github.com/microsoft/TypeScript/pull/12253#issuecomment-263132208
+  slot => {
+    output[slot] = slots[slot].reduce((acc, key) => {
+      if (key) {
+        const utilityClass = getUtilityClass(key);
+        if (utilityClass !== '') {
+          acc.push(utilityClass);
+        }
+        if (classes && classes[key]) {
+          acc.push(classes[key]);
+        }
+      }
+      return acc;
+    }, []).join(' ');
+  });
+  return output;
+}