Generate non-dart assets from “lib” into the build output.

sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart

Index: sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart
diff --git a/sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart b/sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart
index a5098b63eff6fe497211a4f7d44d4369ee3ae041..35bda64f8f3b4ff108974db676b877b82a15fa3f 100644
--- a/sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart
+++ b/sdk/lib/_internal/pub/test/build/includes_assets_from_dependencies_test.dart
@@ -34,11 +34,14 @@ main() {
       ]),
       d.dir("web", [
         d.file("index.html", "html"),
+        d.dir("sub", [
+          d.file("index.html", "html")
+        ])
       ])
     ]).create();
 
     schedulePub(args: ["build", "--all"],
-        output: new RegExp(r"Built 6 files!"));
+        output: new RegExp(r"Built 7 files!"));
 
     d.dir(appPath, [
       d.dir('build', [
@@ -62,6 +65,11 @@ main() {
                 d.file('bar.txt', 'bar'),
               ]),
             ])
+          ]),
+          d.dir("sub", [
+            d.file("index.html", "html"),
+            // "assets" should *only* be created in the top-level directory.
+            d.nothing("assets")
           ])
         ])
       ])