format case test and fix

lib/internal/modules/esm/loader.js

@@ -70,8 +70,8 @@ class Loader {
       );
 
     if (typeof format !== 'string')
-      throw new ERR_INVALID_RETURN_PROPERTY(
-        'module format', 'loader resolve', 'format', format
+      throw new ERR_INVALID_RETURN_PROPERTY_VALUE(
+        'string', 'loader resolve', 'format', format
       );
 
     if (format === 'builtin')

test/es-module/test-esm-loader-invalid-format.mjs

@@ -0,0 +1,11 @@
+// Flags: --experimental-modules --loader ./test/fixtures/es-module-loaders/loader-invalid-format.mjs
+import { expectsError, mustCall } from '../common';
+import assert from 'assert';
+
+import('../fixtures/es-modules/test-esm-ok.mjs')
+.then(assert.fail, expectsError({
+  code: 'ERR_INVALID_RETURN_PROPERTY_VALUE',
+  message: 'Expected string to be returned for the "format" from the ' +
+           '"loader resolve" function but got type undefined.'
+}))
+.then(mustCall());

test/fixtures/es-module-loaders/loader-invalid-format.mjs

@@ -0,0 +1,8 @@
+export async function resolve(specifier, parentModuleURL, defaultResolve) {
+  if (parentModuleURL && specifier === '../fixtures/es-modules/test-esm-ok.mjs') {
+    return {
+      url: 'file:///asdf'
+    };
+  }
+  return defaultResolve(specifier, parentModuleURL);
+}