Fix a typo in pub's urlToAssetId command.

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

Index: sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
index 35be3c3953c537ed5d304d375bbd796647dd74d7..6a2d6b09cf66ee87f1ff7eddc83c8afa9dce7d34 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/web_socket_api.dart
@@ -90,7 +90,7 @@ class WebSocketApi {
   /// This does *not* currently check to ensure the asset actually exists. It
   /// only maps what the corresponding asset *should* be for that URL.
   Map _urlToAssetId(json_rpc.Parameters params) {
-    var url = params["url"].asUrl;
+    var url = params["url"].asUri;
 
     // If a line number was given, map it to the output line.
     var line = params["line"].asIntOr(null);