Remove support for the special "assets" directory.

sdk/lib/_internal/pub/test/barback/directory_args_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS 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 '../serve/utils.dart';
 
 main() {
   initConfig();
 
   setUp(() {
     d.dir(appPath, [
       d.appPubspec(),
-      d.dir('asset', [d.file('file.txt', 'asset')]),
       d.dir('bar', [d.file('file.txt', 'bar')]),
       d.dir('foo', [d.file('file.txt', 'foo')]),
       d.dir('test', [d.file('file.txt', 'test')]),
       d.dir('web', [d.file('file.txt', 'web')])
     ]).create();
   });
 
   integration("builds only the given directories", () {
     schedulePub(args: ["build", "foo", "bar"],
-        output: new RegExp(r'Built 4 files to "build".'));
+        output: new RegExp(r'Built 2 files to "build".'));
 
     d.dir(appPath, [
       d.dir('build', [
         d.dir('bar', [
-          d.file('file.txt', 'bar'),
-          d.dir('assets', [
-            d.dir('myapp', [
-              d.file('file.txt', 'asset')
-            ])
-          ])
+          d.file('file.txt', 'bar')
         ]),
         d.dir('foo', [
-          d.file('file.txt', 'foo'),
-          d.dir('assets', [
-            d.dir('myapp', [
-              d.file('file.txt', 'asset')
-            ])
-          ])
+          d.file('file.txt', 'foo')
         ]),
         d.nothing('test'),
         d.nothing('web')
       ])
     ]).validate();
   });
 
   integration("serves only the given directories", () {
     pubServe(args: ["foo", "bar"]);
 
     requestShouldSucceed("file.txt", "bar", root: "bar");
     requestShouldSucceed("file.txt", "foo", root: "foo");
     expectNotServed("test");
     expectNotServed("web");
 
     endPubServe();
   });
 }