Remove support for the special "assets" directory.

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

Index: sdk/lib/_internal/pub/test/barback/directory_args_test.dart
diff --git a/sdk/lib/_internal/pub/test/barback/directory_args_test.dart b/sdk/lib/_internal/pub/test/barback/directory_args_test.dart
index 59ce17e169898c26e17ad557a5fe1c2e7ad558a8..90426902fdb2153a00d961fdc703ac5a5bdd00c8 100644
--- a/sdk/lib/_internal/pub/test/barback/directory_args_test.dart
+++ b/sdk/lib/_internal/pub/test/barback/directory_args_test.dart
@@ -14,7 +14,6 @@ main() {
   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')]),
@@ -24,25 +23,15 @@ main() {
 
   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')