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app_vue/node_modules/@vue/babel-preset-app/polyfillsPlugin.js

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+const { addSideEffect } = require('@babel/helper-module-imports')
+
+// slightly modifiled from @babel/preset-env/src/utils
+// use an absolute path for core-js modules, to fix conflicts of different core-js versions
+// TODO: remove the `useAbsolutePath` option in v5,
+// because `core-js` is sure to be present in newer projects;
+// we only need absolute path for babel runtime helpers, not for polyfills
+function getModulePath (mod, useAbsolutePath) {
+  const modPath =
+    mod === 'regenerator-runtime'
+      ? 'regenerator-runtime/runtime'
+      : `core-js/modules/${mod}`
+  return useAbsolutePath ? require.resolve(modPath) : modPath
+}
+
+function createImport (path, mod, useAbsolutePath) {
+  return addSideEffect(path, getModulePath(mod, useAbsolutePath))
+}
+
+// add polyfill imports to the first file encountered.
+module.exports = (
+  { types },
+  { polyfills, entryFiles = [], useAbsolutePath }
+) => {
+  return {
+    name: 'vue-cli-inject-polyfills',
+    visitor: {
+      Program (path, state) {
+        if (!entryFiles.includes(state.filename)) {
+          return
+        }
+
+        // imports are injected in reverse order
+        polyfills
+          .slice()
+          .reverse()
+          .forEach(p => {
+            createImport(path, p, useAbsolutePath)
+          })
+      }
+    }
+  }
+}