add scary experimental warning for people who use obscure transformers that are still WIP

src/babel/transformation/file/logger.js

@@ -16,6 +16,10 @@ export default class Logger {
     return parts;
   }
 
+  warn(msg) {
+    console.warn(this._buildMessage(msg));
+  }
+
   error(msg: string, Constructor = Error) {
     throw new Constructor(this._buildMessage(msg));
   }

src/babel/transformation/file/options.json

@@ -47,7 +47,8 @@
   },
 
   "experimental": {
-    "deprecated": "use `--stage 0`/`{ stage: 0 }` instead"
+    "description": "allow use of experimental transformers",
+    "default": false
   },
 
   "highlightCode": {

src/babel/transformation/transformer-pass.js

@@ -13,6 +13,10 @@ export default class TransformerPass {
     this.handlers    = transformer.handlers;
     this.file        = file;
     this.key         = transformer.key;
+
+    if (this.canTransform() && transformer.metadata.experimental && !file.opts.experimental) {
+      file.log.warn(`THE TRANSFORMER ${this.key} HAS BEEN MARKED AS EXPERIMENTAL. USE AT YOUR OWN RISK. THIS WILL HIGHLY LIKELY BREAK YOUR CODE SO USE WITH **EXTREME** CAUTION`);
+    }
   }
 
   canTransform(): boolean {

src/babel/transformation/transformers/minification/constant-folding.js

@@ -2,7 +2,8 @@ import * as t from "../../../types";
 
 export var metadata = {
   optional: true,
-  group: "builtin-prepass"
+  group: "builtin-prepass",
+  experimental: true
 };
 
 export function AssignmentExpression() {

src/babel/transformation/transformers/minification/dead-code-elimination.js

@@ -19,7 +19,8 @@ function toStatements(node) {
 
 export var metadata = {
   optional: true,
-  group: "builtin-pre"
+  group: "builtin-pre",
+  experimental: true
 };
 
 export function ReferencedIdentifier(node, parent, scope) {