Remove support for the special "assets" directory.

sdk/lib/_internal/pub/test/serve/invalid_urls_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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
   integration("responds with a 404 on incomplete special URLs", () {
     d.dir("foo", [
       d.libPubspec("foo", "0.0.1")
     ]).create();
 
     d.dir(appPath, [
       d.appPubspec({
         "foo": {"path": "../foo"}
       }),
-      // Make files that map to the special directory names to ensure they
-      // are *not* found.
-      d.dir("asset", [
-        d.file("packages"),
-        d.file("assets")
-      ]),
       d.dir("lib", [
-        d.file("packages"),
-        d.file("assets")
+        // Make a file that maps to the special "packages" directory to ensure
+        // it is *not* found.
+        d.file("packages")
       ]),
       d.dir("web", [
-        d.file("packages"),
-        d.file("assets")
+        d.file("packages")
       ])
     ]).create();
 
     pubGet();
     pubServe();
     requestShould404("packages");
-    requestShould404("assets");
     requestShould404("packages/");
-    requestShould404("assets/");
     requestShould404("packages/myapp");
-    requestShould404("assets/myapp");
     requestShould404("packages/myapp/");
-    requestShould404("assets/myapp/");
     requestShould404("packages/foo");
-    requestShould404("assets/foo");
     requestShould404("packages/foo/");
-    requestShould404("assets/foo/");
     requestShould404("packages/unknown");
-    requestShould404("assets/unknown");
     requestShould404("packages/unknown/");
-    requestShould404("assets/unknown/");
     endPubServe();
   });
 
 }