fix: transform es modules syntax for Jest when not using Babel

close #1524

packages/@vue/cli-plugin-unit-jest/__tests__/jestPlugin.spec.js

@@ -57,3 +57,13 @@ test('should respect jest.config.js testMatch config', async () => {
   expect(result.stdout).toMatch('testMatch: custom-test-directory/my.spec.js')
   expect(result.stdout).toMatch('No tests found')
 })
+
+test('should work without Babel', async () => {
+  const project = await create('jest-without-babel', {
+    plugins: {
+      '@vue/cli-plugin-unit-jest': {}
+    },
+    useConfigFiles: true
+  })
+  await project.run(`vue-cli-service test:unit`)
+})

packages/@vue/cli-plugin-unit-jest/generator/index.js

@@ -36,8 +36,8 @@ module.exports = api => {
   }
 
   if (!api.hasPlugin('typescript')) {
+    jestConfig.transform['^.+\\.jsx?$'] = 'babel-jest'
     if (api.hasPlugin('babel')) {
-      jestConfig.transform['^.+\\.jsx?$'] = 'babel-jest'
       api.extendPackage({
         devDependencies: {
           'babel-jest': '^23.0.1',
@@ -45,6 +45,12 @@ module.exports = api => {
           'babel-core': '7.0.0-bridge.0'
         }
       })
+    } else {
+      api.extendPackage({
+        babel: {
+          plugins: ['transform-es2015-modules-commonjs']
+        }
+      })
     }
   } else {
     jestConfig.moduleFileExtensions.unshift('ts', 'tsx')

packages/@vue/cli-plugin-unit-jest/package.json

@@ -23,6 +23,8 @@
   },
   "dependencies": {
     "@vue/cli-shared-utils": "^3.0.0-beta.16",
+    "babel-jest": "^23.0.1",
+    "babel-plugin-transform-es2015-modules-commonjs": "^6.26.2",
     "jest": "^23.1.0",
     "jest-serializer-vue": "^1.0.0",
     "jest-transform-stub": "^1.0.0",
@@ -30,7 +32,6 @@
   },
   "devDependencies": {
     "@vue/test-utils": "^1.0.0-beta.16",
-    "babel-jest": "^23.0.1",
     "ts-jest": "^22.4.6"
   }
 }