fix: use @babel/runtime-corejs2 by default

fixes #3186

Author: haoqunjiang

packages/@vue/babel-preset-app/index.js

@@ -142,7 +142,10 @@ module.exports = (context, options = {}) => {
   // transform runtime, but only for helpers
   plugins.push([require('@babel/plugin-transform-runtime'), {
     regenerator: useBuiltIns !== 'usage',
-    corejs: useBuiltIns !== false ? false : 2,
+    // use @babel/runtime-corejs2 so that helpers that need polyfillable APIs will reference core-js instead.
+    // if useBuiltIns is not set to 'usage', then it means users would take care of the polyfills on their own,
+    // i.e., core-js 2 is no longer needed
+    corejs: useBuiltIns === 'usage' ? 2 : false,
     helpers: useBuiltIns === 'usage',
     useESModules: !process.env.VUE_CLI_BABEL_TRANSPILE_MODULES,
     absoluteRuntime: path.dirname(require.resolve('@babel/runtime/package.json'))

packages/@vue/babel-preset-app/package.json

@@ -28,6 +28,7 @@
     "@babel/plugin-transform-runtime": "^7.0.0",
     "@babel/preset-env": "^7.0.0",
     "@babel/runtime": "^7.0.0",
+    "@babel/runtime-corejs2": "^7.2.0",
     "babel-helper-vue-jsx-merge-props": "^2.0.3",
     "babel-plugin-dynamic-import-node": "^2.2.0",
     "babel-plugin-transform-vue-jsx": "^4.0.1",