Remove support for the special "assets" directory.

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

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index f097e9f4148cedd921f4c9f7b67bf4f545ecd393..cea9ca279d391c3b3fc0f901608c3bcc4baec72c 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -570,7 +570,7 @@ class AssetEnvironment {
       // Don't watch files symlinked into these directories.
       // TODO(rnystrom): If pub gets rid of symlinks, remove this.
       var parts = path.split(event.path);
-      if (parts.contains("packages") || parts.contains("assets")) return;
+      if (parts.contains("packages")) return;
 
       // Skip files that were (most likely) compiled from nearby ".dart"
       // files. These are created by the Editor's "Run as JavaScript"