fix(babel): set useBuiltins to false during MODERN_BUILD (#1758)

* fix(babel): set useBuiltins to false dring MODERN_BUILD

* refactor(babel): use `ignoreBrowserslistConfig`
...instead of deactivating `useBuiltins`.
Leaves possibility for future polyfills open.

* fix: don't override user options

Author: LinusBorg

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

@@ -35,6 +35,38 @@ test('polyfill detection', () => {
   expect(code).toMatch(`import "core-js/modules/es6.map"`)
 })
 
+test('modern mode always skips polyfills', () => {
+  process.env.VUE_CLI_MODERN_BUILD = true
+  let { code } = babel.transformSync(`
+    const a = new Map()
+  `.trim(), {
+    babelrc: false,
+    presets: [[preset, {
+      targets: { ie: 9 },
+      useBuiltIns: 'usage'
+    }]]
+  })
+  // 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"`)
+
+  ;({ code } = babel.transformSync(`
+    const a = new Map()
+  `.trim(), {
+    babelrc: false,
+    presets: [[preset, {
+      targets: { ie: 9 },
+      useBuiltIns: 'entry'
+    }]]
+  }))
+  // 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"`)
+  delete process.env.VUE_CLI_MODERN_BUILD
+})
+
 test('object spread', () => {
   const { code } = babel.transformSync(`
     const a = { ...b }

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

@@ -42,7 +42,7 @@ module.exports = (context, options = {}) => {
     modules = false,
     targets: rawTargets,
     spec,
-    ignoreBrowserslistConfig,
+    ignoreBrowserslistConfig = !!process.env.VUE_CLI_MODERN_BUILD,
     configPath,
     include,
     exclude,