Made the return value of resolver functions optional. Uses a fallback on failure.

src/Bundle.js

@@ -1,5 +1,6 @@
 import { Promise } from 'sander';
 import MagicString from 'magic-string';
+import attempt from './utils/attempt.js';
 import { blank, keys } from './utils/object';
 import Module from './Module';
 import ExternalModule from './ExternalModule';
@@ -17,12 +18,16 @@ export default class Bundle {
 		this.entry = options.entry;
 		this.entryModule = null;
 
-		this.resolveId = options.resolveId || defaultResolver;
-		this.load = options.load || defaultLoader;
+		this.resolveId = ensureArray( options.resolveId )
+			.reduce( attempt, defaultResolver );
+
+		this.load = ensureArray( options.load )
+			.reduce( attempt, defaultLoader );
 
 		this.resolveOptions = {
 			external: ensureArray( options.external ),
-			resolveExternal: options.resolveExternal || defaultExternalResolver
+			resolveExternal: ensureArray( options.resolveExternal )
+				.reduce( attempt, defaultExternalResolver )
 		};
 
 		this.loadOptions = {

src/utils/attempt.js

@@ -0,0 +1,11 @@
+// Given a `fallback` and a `preferred` function,
+// return a function that attempts to call the `preferred`.
+// If it fails, or returns undefined, call the fallback and return its value.
+export default function attempt ( fallback, preferred ) {
+	return function ( ...args ) {
+		const boundFallback = () => fallback( ...args );
+
+		return Promise.resolve( preferred( ...args ) )
+			.then( res => res === undefined ? boundFallback() : res, boundFallback );
+	};
+}

test/function/custom-path-resolver-on-entry/_config.js

@@ -18,8 +18,6 @@ module.exports = {
 			if ( importer[0] === '@' ) {
 				return path.resolve( __dirname, importee ) + '.js';
 			}
-
-			return path.resolve( path.dirname( importer ), importee ) + '.js';
 		},
 		load: function ( moduleId ) {
 			if ( moduleId[0] === '@' ) {