provide informative error if rollup is used in browser without custom resolveId/load functions

src/utils/defaults.js

@@ -16,6 +16,8 @@ function addJsExtensionIfNecessary ( file ) {
 }
 
 export function resolveId ( importee, importer ) {
+	if ( typeof process === 'undefined' ) throw new Error( `It looks like you're using Rollup in a non-Node.js environment. This means you must supply a plugin with custom resolveId and load functions. See https://github.com/rollup/rollup/wiki/Plugins for more information` );
+
 	// absolute paths are left untouched
 	if ( isAbsolute( importee ) ) return addJsExtensionIfNecessary( importee );