[preset-env] Don't use async-to-generator when already using re… (#9481)

* [preset-env] Don't use async-to-generator when already using regenerator

* Add tests

* Update fixtures

packages/babel-plugin-transform-classes/test/fixtures/regression/T6755/output.js

@@ -1,7 +1,3 @@
-function asyncGeneratorStep(gen, resolve, reject, _next, _throw, key, arg) { try { var info = gen[key](arg); var value = info.value; } catch (error) { reject(error); return; } if (info.done) { resolve(value); } else { Promise.resolve(value).then(_next, _throw); } }
-
-function _asyncToGenerator(fn) { return function () { var self = this, args = arguments; return new Promise(function (resolve, reject) { var gen = fn.apply(self, args); function _next(value) { asyncGeneratorStep(gen, resolve, reject, _next, _throw, "next", value); } function _throw(err) { asyncGeneratorStep(gen, resolve, reject, _next, _throw, "throw", err); } _next(undefined); }); }; }
-
 var Example =
 /*#__PURE__*/
 function () {
@@ -11,33 +7,21 @@ function () {
 
   var _proto = Example.prototype;
 
-  _proto.test1 =
-  /*#__PURE__*/
-  function () {
-    var _test = _asyncToGenerator(
-    /*#__PURE__*/
-    regeneratorRuntime.mark(function _callee() {
-      return regeneratorRuntime.wrap(function _callee$(_context) {
-        while (1) {
-          switch (_context.prev = _context.next) {
-            case 0:
-              _context.next = 2;
-              return Promise.resolve(2);
+  _proto.test1 = function test1() {
+    return regeneratorRuntime.async(function test1$(_context) {
+      while (1) {
+        switch (_context.prev = _context.next) {
+          case 0:
+            _context.next = 2;
+            return regeneratorRuntime.awrap(Promise.resolve(2));
 
-            case 2:
-            case "end":
-              return _context.stop();
-          }
+          case 2:
+          case "end":
+            return _context.stop();
         }
-      }, _callee);
-    }));
-
-    function test1() {
-      return _test.apply(this, arguments);
-    }
-
-    return test1;
-  }();
+      }
+    });
+  };
 
   _proto.test2 =
   /*#__PURE__*/