process: add emitExperimentalWarning()

This adds process.emitExperimentalWarning() which can used to
communicate to our users that a feature they are using is experimental
and can be changed/removed at any time.

Ref: nodejs#9036

lib/internal/process/warning.js

@@ -4,6 +4,8 @@ const prefix = `(${process.release.name}:${process.pid}) `;
 
 exports.setup = setupProcessWarnings;
 
+const experimentalWarnings = new Set();
+
 function setupProcessWarnings() {
   if (!process.noProcessWarnings) {
     process.on('warning', (warning) => {
@@ -46,4 +48,12 @@ function setupProcessWarnings() {
     }
     process.nextTick(() => process.emit('warning', warning));
   };
+
+  process.emitExperimentalWarning = function(feature) {
+    if (experimentalWarnings.has(feature)) return;
+    experimentalWarnings.add(feature);
+    const msg = `${feature} is an experimental feature. ` +
+                'This feature could change at any time.';
+    process.emitWarning(msg, 'ExperimentalWarning');
+  };
 }

test/parallel/test-process-emit-experimentalwarning.js

@@ -0,0 +1,9 @@
+'use strict';
+
+const common = require('../common');
+
+const warning = 'new_feature is an experimental feature. This feature could ' +
+                'change at any time.';
+common.expectWarning('ExperimentalWarning', warning);
+process.emitExperimentalWarning('new_feature');
+process.emitExperimentalWarning('new_feature');