Remove support for the special "assets" directory.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index 101669ff421e8e191c0e43d8179d74e44bc07040..b3b179721a819fec5074e022ef8158ccda4dcff9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -384,8 +384,8 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
   }
 
   AssetId _sourceUrlToId(Uri url) {
-    // See if it's a special path with "packages" or "assets" in it.
-    var id = specialUrlToId(url);
+    // See if it's a package path.
+    var id = packagesUrlToId(url);
     if (id != null) return id;
 
     // See if it's a path to a "public" asset within the root package. All