Add test to make sure .d.ts files are ignored when checking for TypeScript project (facebook#6858)

Author: iansu

test/fixtures/issue-5947-not-typescript/index.test.js

@@ -16,6 +16,7 @@ test('Ignores node_modules when detecting TypeScript', async () => {
     'package',
     'index.ts',
   ];
+  const dtsSrcPath = [testSetup.testDirectory, 'src', 'types', 'index.d.ts'];
   const tsSrcPath = path.join(testSetup.testDirectory, 'src', 'index.ts');
 
   // Step 1.
@@ -28,6 +29,13 @@ test('Ignores node_modules when detecting TypeScript', async () => {
   await testSetup.scripts.build();
   expect(fs.existsSync(tsConfigPath)).toBe(false);
 
+  // Step 1b.
+  // See if src/types/index.d.ts is treated as a JS project
+  fs.mkdirSync(path.join(...dtsSrcPath.slice(0, 3)));
+  fs.writeFileSync(path.join(...dtsSrcPath));
+  await testSetup.scripts.build();
+  expect(fs.existsSync(tsConfigPath)).toBe(false);
+
   // Step 2.
   // Add TS and ensure tsconfig.json is generated
   fs.writeFileSync(tsSrcPath);