add dfs tests for https://github.com/tc39/ecma262/pull/1669

test/language/module-code/verify-dfs-a_FIXTURE.js

@@ -0,0 +1,8 @@
+// Copyright (C) 2020 Ecma International. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+import { evaluated, check } from './verify-dfs.js';
+
+check(import('./verify-dfs-b_FIXTURE.js'));
+
+evaluated('A');

test/language/module-code/verify-dfs-b_FIXTURE.js

@@ -0,0 +1,6 @@
+// Copyright (C) 2020 Ecma International. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+import { evaluated } from './verify-dfs.js';
+
+evaluated('B');

test/language/module-code/verify-dfs.js

@@ -0,0 +1,34 @@
+// Copyright (C) 2020 Ecma International. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Dynamic import can't preempt DFS evaluation order
+esid: sec-moduleevaluation
+info: |
+  Evaluate ( ) Concrete Method
+
+  1. Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within the surrounding agent.
+  [...]
+flags: [module, async]
+features: [dynamic-import]
+---*/
+
+import './verify-dfs-a_FIXTURE.js';
+import './verify-dfs-b_FIXTURE.js';
+
+// rely on function hoisting to create shared array
+export function evaluated(name) {
+  if (!evaluated.order) {
+    evaluated.order = [];
+  }
+  evaluated.order.push(name);
+}
+
+export function check(promise) {
+  promise.then(() => {
+    assert.sameValue(evaluated.order.length, 2);
+    assert.sameValue(evaluated.order[0], 'A');
+    assert.sameValue(evaluated.order[1], 'B');
+  })
+  .then($DONE, $DONE);
+}