Add option to warn at runtime when non-existent properties are accessed on css module

Author: Andrew Levine

lib/loader.js

@@ -88,7 +88,16 @@ module.exports = function(content, map) {
 
 		var exportJs = compileExports(result, importItemMatcher.bind(this), camelCaseKeys);
 		if (exportJs) {
-			exportJs = "exports.locals = " + exportJs + ";";
+			if (query.cssModuleDevWarnings) {
+				exportJs = "exports.locals = new Proxy(" + exportJs + "," +
+					"{ get(target, property) { " +
+					"if (typeof target[property] !== 'string' && property !== '__esModule') {" +
+					"console.warn(`Attempted to access non-existent className '${property} on CSS module'`)" +
+					"} else { return target[property] }" +
+					"}})";
+			} else {
+				exportJs = "exports.locals = " + exportJs + ";"
+			}
 		}
 
 		var moduleJs;