internal: add emitExperimentalWarning function

Adds a new emitExperimentalWarning function to internal/util to warn
users that they are using an experimental feature and that feature can
change at any time.

Refs: nodejs#9036

lib/internal/util.js

@@ -15,6 +15,8 @@ const {
 
 const noCrypto = !process.versions.openssl;
 
+const experimentalWarnings = new Set();
+
 function isError(e) {
   return objectToString(e) === '[object Error]' || e instanceof Error;
 }
@@ -120,6 +122,14 @@ function normalizeEncoding(enc) {
   }
 }
 
+function emitExperimentalWarning(feature) {
+  if (experimentalWarnings.has(feature)) return;
+  const msg = `${feature} is an experimental feature. This feature could ` +
+       'change at any time';
+  experimentalWarnings.add(feature);
+  process.emitWarning(msg, 'ExperimentalWarning');
+}
+
 function filterDuplicateStrings(items, low) {
   const map = new Map();
   for (var i = 0; i < items.length; i++) {
@@ -290,6 +300,7 @@ module.exports = {
   createClassWrapper,
   decorateErrorStack,
   deprecate,
+  emitExperimentalWarning,
   filterDuplicateStrings,
   getConstructorOf,
   isError,