Skip plugin for non-Flow JS code (facebook#1556)

Summary:

This is a mitigation for facebook#1549.

Updates `babel-plugin-syntax-hermes-parser` to abort when the file contents do not include `flow`.

**Context: React Native**

Originally changed in facebook/react-native#46696, as our internal Flow support had diverged from `babel/plugin-syntax-flow` (facebook/react-native#46601).

We effectively have three flavours of JavaScript in support:
- Flow@latest for the `react-native` package, shipped as source — uses `hermes-parser`.
- TypeScript for product code (community template, Expo) — uses `babel/plugin-syntax-typescript`.
- Plain JavaScript/JSX in product code, *which may be extended with additional user Babel plugins and needs lenient parsing* — uses base `babel/parser` (**this change**).

I'd love to simplify this 😅.

Reviewed By: cipolleschi

Differential Revision: D65272155

tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js

@@ -16,7 +16,7 @@ import {transformSync} from '@babel/core';
 import hermesParserPlugin from '../src';
 import * as HermesParser from 'hermes-parser';
 
-const MODULE_PREAMBLE = '"use strict";\n\n';
+const MODULE_PREAMBLE = '// @flow\n\n"use strict";\n\n';
 
 describe('babel-plugin-syntax-hermes-parser', () => {
   test('test basic parsing', () => {
@@ -37,7 +37,9 @@ describe('babel-plugin-syntax-hermes-parser', () => {
       filename: 'foo.ts',
     });
     expect(output.code).toMatchInlineSnapshot(`
-      ""use strict";
+      "// @flow
+
+      "use strict";
 
       const a = 1;"
     `);
@@ -54,7 +56,9 @@ describe('babel-plugin-syntax-hermes-parser', () => {
       },
     });
     expect(output.code).toMatchInlineSnapshot(`
-      ""use strict";
+      "// @flow
+
+      "use strict";
 
       function Foo() {}"
     `);

tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js

@@ -37,8 +37,9 @@ export default function BabelPluginSyntaxHermesParser(
     parserOverride(code: string) {
       const filename = curFilename;
       if (
-        filename != null &&
-        (filename.endsWith('.ts') || filename.endsWith('.tsx'))
+        !/@flow/.test(code) ||
+        (filename != null &&
+          (filename.endsWith('.ts') || filename.endsWith('.tsx')))
       ) {
         return;
       }