Update pub to look for VM's new error message text.

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

Index: sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart b/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
index bfd9f66f1ed668f87a0193ffb0d45a69be829149..65ba68f72ebe588c95c790b570093f8c6cfd97ba 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/transformer_isolate.dart
@@ -93,8 +93,9 @@ class TransformerIsolate {
         // "package:" URI, so we have to be liberal in what we look for in the
         // error message.
         var missingTransformer = idsToUrls.keys.firstWhere((id) =>
-            firstErrorLine.startsWith("Uncaught Error: Failure getting ") &&
-              firstErrorLine.contains(idsToUrls[id].path),
+            firstErrorLine.startsWith(
+                "Uncaught Error: Load Error: Failure getting ") &&
+            firstErrorLine.contains(idsToUrls[id].path),
             orElse: () => throw error);
         var packageUri = idToPackageUri(idsToAssetIds[missingTransformer]);