fix: skip polyfills for modern mode and fix tests

Author: haoqunjiang

packages/@vue/babel-preset-app/__tests__/babel-preset.spec.js

@@ -14,7 +14,7 @@ test('polyfill detection', () => {
       targets: { node: 'current' }
     }]]
   })
-  // default includes
+  // default i  ncludes
   expect(code).not.toMatch(`import "core-js/modules/es6.promise"`)
   // usage-based detection
   expect(code).not.toMatch(`import "core-js/modules/es6.map"`)
@@ -32,7 +32,7 @@ test('polyfill detection', () => {
   // promise polyfill alone doesn't work in IE, needs this as well. fix: #1642
   expect(code).toMatch(`import "core-js/modules/es6.array.iterator"`)
   // usage-based detection
-  expect(code).toMatch(`import "core-js/modules/es6.map"`)
+  expect(code).toMatch(/import _Map from ".*@babel\/runtime-corejs2\/core-js\/map"/)
 })
 
 test('modern mode always skips polyfills', () => {
@@ -49,7 +49,7 @@ test('modern mode always skips polyfills', () => {
   // default includes
   expect(code).not.toMatch(`import "core-js/modules/es6.promise"`)
   // usage-based detection
-  expect(code).not.toMatch(`import "core-js/modules/es6.map"`)
+  expect(code).not.toMatch(/import _Map from ".*@babel\/runtime-corejs2\/core-js\/map"/)
 
   ;({ code } = babel.transformSync(`
     const a = new Map()
@@ -63,7 +63,7 @@ test('modern mode always skips polyfills', () => {
   // default includes
   expect(code).not.toMatch(`import "core-js/modules/es6.promise"`)
   // usage-based detection
-  expect(code).not.toMatch(`import "core-js/modules/es6.map"`)
+  expect(code).not.toMatch(/import _Map from ".*@babel\/runtime-corejs2\/core-js\/map"/)
   delete process.env.VUE_CLI_MODERN_BUILD
 })
 
@@ -92,7 +92,7 @@ test('async/await', () => {
   // should use regenerator runtime
   expect(code).toMatch(`import "regenerator-runtime/runtime"`)
   // should use required helper instead of inline
-  expect(code).toMatch(/@babel.*runtime\/helpers\/.*asyncToGenerator/)
+  expect(code).toMatch(/import _asyncToGenerator from ".*@babel\/runtime-corejs2\/helpers\/esm\/asyncToGenerator\"/)
 })
 
 test('jsx', () => {

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

@@ -144,8 +144,8 @@ module.exports = (context, options = {}) => {
     regenerator: useBuiltIns !== 'usage',
     // 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,
+    // i.e., core-js 2 is no longer needed.
+    corejs: (useBuiltIns === 'usage' && !process.env.VUE_CLI_MODERN_BUILD) ? 2 : false,
     helpers: useBuiltIns === 'usage',
     useESModules: !process.env.VUE_CLI_BABEL_TRANSPILE_MODULES,
     absoluteRuntime: path.dirname(require.resolve('@babel/runtime/package.json'))