Warn when serving an asset from "asset".

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

Index: sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
index 611d62ff1931fc51a014003636d0b910dc222f19..954658b600eb797c3360b84674d77538fd9b4ca5 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/barback_server.dart
@@ -117,6 +117,25 @@ class BarbackServer extends BaseServer<BarbackServerResult> {
       return;
     }
 
+    // TODO(rnystrom): Remove this when #16647 is fixed.
+    // The "assets" path is deprecated so warn if the user is relying on it.
+    // We do this here in pub serve so we only warn if they in fact actually
+    // use an asset from a package's "asset" directory.
+    if (id.path.startsWith("asset/")) {
+      var message = 'Warning: Support for the "asset" directory is deprecated '
+          'and will be removed soon.\n';
+
+      var fixed = id.path.replaceAll(new RegExp(r"^asset/"), "lib/");
+      if (id.package == environment.rootPackage.name) {
+        message += 'Please move "${id.path}" to "$fixed".';
+      } else {
+        message += 'Please ask the maintainer of "${id.package}" to move '
+            '"${id.path}" to "$fixed".';
+      }
+
+      log.warning(log.yellow(message));
+    }
+
     logRequest(request, "Loading $id");
     environment.barback.getAssetById(id).then((result) {
       logRequest(request, "getAssetById($id) returned");