Use async-to-generator even when regenerator is enabled (#10839)

While it isn't technically needed because regenerator already handles
async functions, it doesn't play well with the Promise detection logic
used by @babel/preset-env's useBuiltIns and @babel/transform-runtime.

The plugin exclusion logic isn't removed, because
1) it will be useful when merging @babel/preset-modules
    into @babel/preset-env
2) if/when regenerator will support injecting a custom promise,
    we can enable this optimization again

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

@@ -1,3 +1,7 @@
+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 () {
@@ -7,21 +11,33 @@ function () {
 
   var _proto = Example.prototype;
 
-  _proto.test1 = function test1() {
-    return regeneratorRuntime.async(function test1$(_context) {
-      while (1) {
-        switch (_context.prev = _context.next) {
-          case 0:
-            _context.next = 2;
-            return regeneratorRuntime.awrap(Promise.resolve(2));
+  _proto.test1 =
+  /*#__PURE__*/
+  function () {
+    var _test = _asyncToGenerator(
+    /*#__PURE__*/
+    regeneratorRuntime.mark(function _callee() {
+      return regeneratorRuntime.wrap(function _callee$(_context) {
+        while (1) {
+          switch (_context.prev = _context.next) {
+            case 0:
+              _context.next = 2;
+              return Promise.resolve(2);
 
-          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__*/