Fix Flow errors in globalEvalWithSourceUrl Fantom test

Use unknown instead of the deprecated mixed, and read the host function
untyped in the arity test so Flow does not reject the deliberate bad call.

Author: abzokhattab

packages/react-native/Libraries/Core/Devtools/__tests__/globalEvalWithSourceUrl-itest.js

@@ -12,7 +12,7 @@ import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
 
 const SOURCE_URL = 'globalEvalWithSourceUrl-itest.bundle';
 
-function getHelper(): (code: string, sourceUrl?: string) => mixed {
+function getHelper(): (code: string, sourceUrl?: string) => unknown {
   // $FlowFixMe[prop-missing]
   const helper = global.globalEvalWithSourceUrl;
   if (typeof helper !== 'function') {
@@ -23,7 +23,7 @@ function getHelper(): (code: string, sourceUrl?: string) => mixed {
   return helper;
 }
 
-function getStack(fn: () => mixed): string {
+function getStack(fn: () => unknown): string {
   try {
     fn();
   } catch (e) {
@@ -71,8 +71,11 @@ describe('globalEvalWithSourceUrl', () => {
   });
 
   it('rejects an invalid argument count', () => {
-    const helper = getHelper();
-    // $FlowFixMe[incompatible-call]
+    // Read it untyped on purpose: this test calls the host function with an
+    // arity the typed wrapper would not allow.
+    const helper: (...args: Array<unknown>) => unknown =
+      // $FlowFixMe[prop-missing]
+      global.globalEvalWithSourceUrl;
     expect(() => helper()).toThrow(
       'globalEvalWithSourceUrl arg count must be 1 or 2',
     );