Make a slightly better 404 page for pub serve.

sdk/lib/_internal/pub/test/serve/404_page_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library pub_tests;
+
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+
+main() {
+  initConfig();
+  setUp(() {
+    d.dir(appPath, [
+      d.appPubspec()
+    ]).create();
+  });
+
+  integration("the 404 page describes the missing asset", () {
+    pubServe();
+
+    scheduleRequest("packages/foo/missing.txt").then((response) {
+      expect(response.statusCode, equals(404));
+
+      // Should mention the asset that can't be found.
+      expect(response.body, contains("foo"));
+      expect(response.body, contains("missing.txt"));
+    });
+
+    endPubServe();
+  });
+
+  integration("the 404 page describes the error", () {
+    pubServe();
+
+    scheduleRequest("packages").then((response) {
+      expect(response.statusCode, equals(404));
+
+      // Should mention the asset that can't be found.
+      expect(response.body, contains('"/packages"'));
+    });
+
+    endPubServe();
+  });
+}