Add core-js as valid polyfill source (#7315)

packages/babel-preset-env/README.md

@@ -342,6 +342,8 @@ import "core-js/modules/es7.string.pad-start";
 import "core-js/modules/es7.string.pad-end";
 ```
 
+This will also work for `core-js` directly (`import "core-js";` or `require('core-js');`)
+
 #### `useBuiltIns: false`
 
 Don't add polyfills automatically per file, or transform `import "@babel/polyfill"` to individual polyfills.

packages/babel-preset-env/src/utils.js

@@ -86,7 +86,7 @@ export const filterStageFromList = (list: any, stageList: any) => {
 };
 
 export const isPolyfillSource = (source: string): boolean =>
-  source === "@babel/polyfill";
+  source === "@babel/polyfill" || source === "core-js";
 
 export const isRequire = (t: Object, path: Object): boolean =>
   t.isExpressionStatement(path.node) &&

packages/babel-preset-env/test/fixtures/preset-options/core-js/input.js

@@ -0,0 +1 @@
+import "core-js";

packages/babel-preset-env/test/fixtures/preset-options/core-js/options.json

@@ -0,0 +1,11 @@
+{
+  "presets": [
+    ["../../../../lib", {
+      "targets": {
+        "chrome": 55
+      },
+      "modules": false,
+      "useBuiltIns": "entry"
+    }]
+  ]
+}

packages/babel-preset-env/test/fixtures/preset-options/core-js/output.js

@@ -0,0 +1,6 @@
+import "core-js/modules/es6.array.sort";
+import "core-js/modules/es7.string.pad-start";
+import "core-js/modules/es7.string.pad-end";
+import "core-js/modules/web.timers";
+import "core-js/modules/web.immediate";
+import "core-js/modules/web.dom.iterable";

packages/babel-preset-env/test/fixtures/preset-options/ios-6/output.js

@@ -1 +0,0 @@
-import "core-js";