fix: should not inject babel-core shim if babel plugin is not used

close #1424

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

@@ -36,14 +36,16 @@ module.exports = api => {
   }
 
   if (!api.hasPlugin('typescript')) {
-    jestConfig.transform['^.+\\.jsx?$'] = 'babel-jest'
-    api.extendPackage({
-      devDependencies: {
-        'babel-jest': '^22.4.3',
-        // this is for now necessary to force babel-jest and vue-jest to use babel 7
-        'babel-core': '7.0.0-bridge.0'
-      }
-    })
+    if (api.hasPlugin('babel')) {
+      jestConfig.transform['^.+\\.jsx?$'] = 'babel-jest'
+      api.extendPackage({
+        devDependencies: {
+          'babel-jest': '^22.4.3',
+          // this is for now necessary to force babel-jest and vue-jest to use babel 7
+          'babel-core': '7.0.0-bridge.0'
+        }
+      })
+    }
   } else {
     jestConfig.moduleFileExtensions.unshift('ts', 'tsx')
     jestConfig.transform['^.+\\.tsx?$'] = 'ts-jest'