build: make 'npm install' work reliably and issue build warning when node_modules look stale

This is done a in bit kludgy way on purpose so that it works on Windows and CI.

Works around npm/npm#8452.

Closes #2038

tools/npm/check-node-modules.js

@@ -0,0 +1,50 @@
+"use strict";
+
+var fs = require('fs');
+var path = require('path');
+
+var NPM_SHRINKWRAP_FILE = 'npm-shrinkwrap.json';
+var NPM_SHRINKWRAP_CACHED_FILE = 'node_modules/npm-shrinkwrap.cached.json';
+var FS_OPTS = {encoding: 'utf-8'};
+var PROJECT_ROOT = path.join(__dirname, '../../');
+
+
+function checkNodeModules(logOutput, purgeIfStale) {
+  var nodeModulesOK = _checkCache(NPM_SHRINKWRAP_FILE, NPM_SHRINKWRAP_CACHED_FILE);
+
+  if (nodeModulesOK) {
+    if (logOutput) console.log(':-) npm dependencies are looking good!');
+  } else {
+    if (logOutput) console.error(':-( npm dependencies are stale or in an in unknown state!');
+    if (purgeIfStale) {
+      if (logOutput) console.log('    purging...');
+
+      var nodeModulesPath = path.join(PROJECT_ROOT, 'node_modules');
+
+      if (fs.existsSync(nodeModulesPath)) {
+        // lazy-load fs-extra
+        var fse = require('fs-extra');
+        fse.removeSync(nodeModulesPath);
+      }
+    }
+  }
+
+  return nodeModulesOK;
+}
+
+
+function _checkCache(markerFile, cacheMarkerFile) {
+  var absoluteMarkerFilePath = path.join(PROJECT_ROOT, markerFile);
+  var absoluteCacheMarkerFilePath = path.join(PROJECT_ROOT, cacheMarkerFile);
+
+
+  if (!fs.existsSync(absoluteCacheMarkerFilePath)) return false;
+
+  var markerContent = fs.readFileSync(absoluteMarkerFilePath, FS_OPTS);
+  var cacheMarkerContent = fs.readFileSync(absoluteCacheMarkerFilePath, FS_OPTS);
+
+  return markerContent == cacheMarkerContent;
+}
+
+
+module.exports = checkNodeModules;