Provide and respect HTTP cache headers in pub serve.

test/serve/caches_unchanged_assets.dart

+// Copyright (c) 2016, 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.
+
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+
+main() {
+  integration("serves and responds to cache headers", () {
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir("web", [
+        d.file("file.txt", "stuff"),
+      ])
+    ]).create();
+
+    pubGet();
+    pubServe();
+
+    var etag;
+    schedule(() async {
+      var response = await scheduleRequest("file.txt");
+      expect(response.statusCode, equals(200));
+      expect(response.body, equals("stuff"));
+      etag = response.headers["etag"];
+    });
+
+    schedule(() async {
+      var response = await scheduleRequest("file.txt",
+          headers: {"if-none-match": etag});
+      expect(response.statusCode, equals(304));
+      expect(response.headers["etag"], equals(etag));
+      expect(response.body, equals(""));
+    });
+
+    endPubServe();
+  });
+}