Use enum for Fantom modes (facebook#48179)

Summary:
Pull Request resolved: facebook#48179

Changelog: [internal]

Migrating this type to an enum, which is safer, because it prevents errors like:

```
// when it's actually 'dev'
if (mode === 'development') {

}
```

Reviewed By: rshest

Differential Revision: D66888985

fbshipit-source-id: 4f3f91fad6ca5256baa2123425b2bad11fe036f9

packages/react-native-fantom/runner/getFantomTestConfig.js

@@ -19,7 +19,10 @@ type JsOnlyFeatureFlags = (typeof ReactNativeFeatureFlags)['jsOnly'];
 
 type DocblockPragmas = {[key: string]: string | string[]};
 
-export type FantomTestConfigMode = 'dev' | 'opt';
+export enum FantomTestConfigMode {
+  Development,
+  Optimized,
+}
 
 export type FantomTestConfigCommonFeatureFlags = Partial<{
   [key in keyof CommonFeatureFlags]: CommonFeatureFlags[key]['defaultValue'],
@@ -37,7 +40,7 @@ export type FantomTestConfig = {
   },
 };
 
-const DEFAULT_MODE: FantomTestConfigMode = 'dev';
+const DEFAULT_MODE: FantomTestConfigMode = FantomTestConfigMode.Development;
 
 const FANTOM_FLAG_FORMAT = /^(\w+):(\w+)$/;
 
@@ -84,10 +87,15 @@ export default function getFantomTestConfig(
 
     const mode = maybeMode;
 
-    if (mode === 'dev' || mode === 'opt') {
-      config.mode = mode;
-    } else {
-      throw new Error(`Invalid Fantom mode: ${mode}`);
+    switch (mode) {
+      case 'dev':
+        config.mode = FantomTestConfigMode.Development;
+        break;
+      case 'opt':
+        config.mode = FantomTestConfigMode.Optimized;
+        break;
+      default:
+        throw new Error(`Invalid Fantom mode: ${mode}`);
     }
   }
 

packages/react-native-fantom/runner/runner.js

@@ -13,6 +13,7 @@ import type {TestSuiteResult} from '../runtime/setup';
 
 import entrypointTemplate from './entrypoint-template';
 import getFantomTestConfig from './getFantomTestConfig';
+import {FantomTestConfigMode} from './getFantomTestConfig';
 import {
   getBuckModeForPlatform,
   getDebugInfoFromCommandResult,
@@ -97,7 +98,7 @@ module.exports = async function runTest(
 
   const testConfig = getFantomTestConfig(testPath);
 
-  const isOptimizedMode = testConfig.mode === 'opt';
+  const isOptimizedMode = testConfig.mode === FantomTestConfigMode.Optimized;
 
   const metroConfig = await Metro.loadConfig({
     config: path.resolve(__dirname, '..', 'config', 'metro.config.js'),