Add legacy option to plugin-proposal-decorators.

packages/babel-plugin-syntax-decorators/README.md

@@ -33,3 +33,11 @@ require("@babel/core").transform("code", {
   plugins: ["@babel/plugin-syntax-decorators"]
 });
 ```
+
+## Options
+
+### `legacy`
+
+`boolean`, defaults to `false`.
+
+Use the legacy (stage 1) decorators syntax.

packages/babel-plugin-syntax-decorators/src/index.js

@@ -1,11 +1,16 @@
 import { declare } from "@babel/helper-plugin-utils";
 
-export default declare(api => {
+export default declare((api, options) => {
   api.assertVersion(7);
 
+  const { legacy = false } = options;
+  if (typeof legacy !== "boolean") {
+    throw new Error("'legacy' must be a boolean.");
+  }
+
   return {
     manipulateOptions(opts, parserOpts) {
-      parserOpts.plugins.push("decorators");
+      parserOpts.plugins.push(legacy ? "decorators" : "decorators2");
     },
   };
 });