[babel 8] Use the JSX automatic runtime by default (#12630)

Co-authored-by: Nicolò Ribaudo <nicolo.ribaudo@gmail.com>
Co-authored-by: Arun Kumar Mohan <arunmohandm@gmail.com>

packages/babel-plugin-transform-react-jsx/test/fixtures/spread-transform/transform-to-babel-extend/options.json

@@ -1,6 +1,7 @@
 {
-  "plugins": ["transform-react-jsx", ["proposal-object-rest-spread", {
-    "loose": true,
-    "useBuiltIns": false
-  }], "external-helpers"]
+  "plugins": [
+    ["transform-react-jsx", { "runtime": "classic" }],
+    ["proposal-object-rest-spread", { "loose": true, "useBuiltIns": false }],
+    "external-helpers"
+  ]
 }

packages/babel-plugin-transform-react-jsx/test/fixtures/spread-transform/transform-to-object-assign/output.js

@@ -1,3 +1,5 @@
-var div = /*#__PURE__*/React.createElement(Component, Object.assign({}, props, {
+var _reactJsxRuntime = require("react/jsx-runtime");
+
+var div = /*#__PURE__*/_reactJsxRuntime.jsx(Component, Object.assign({}, props, {
   foo: "bar"
 }));