Add rollup-plugin-visualizer, but leave commented out

(this requires node 8+)

build_src.js

@@ -6,6 +6,7 @@ const json = require('rollup-plugin-json');
 const nodeResolve = require('rollup-plugin-node-resolve');
 const rollup = require('rollup');
 const shell = require('shelljs');
+// const visualizer = require('rollup-plugin-visualizer');
 
 
 module.exports = function buildSrc() {
@@ -16,6 +17,7 @@ module.exports = function buildSrc() {
 
         // Start clean
         shell.rm('-f', [
+            'docs/statistics.html',
             'dist/iD.js',
             'dist/iD.js.map'
         ]);
@@ -42,6 +44,11 @@ module.exports = function buildSrc() {
                     }),
                     commonjs(),
                     json({ indent: '' })
+                    // uncomment when we require node 8+
+                    // visualizer({
+                    //     filename: 'docs/statistics.html',
+                    //     sourcemap: true
+                    // })
                 ]
             })
             .then(function (bundle) {

package.json

@@ -86,6 +86,7 @@
     "rollup-plugin-includepaths": "~0.2.3",
     "rollup-plugin-json": "^4.0.0",
     "rollup-plugin-node-resolve": "^4.0.0",
+    "rollup-plugin-visualizer": "^1.1.0",
     "shelljs": "^0.8.0",
     "shx": "^0.3.0",
     "sinon": "7.2.7",