Update pub's isolate load error parsing code.

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

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 2d5d8cb23625855e7ade9a90e89e67da094810eb..1a6c2cb13c0993cc4bcf57b3f8c7cd92d40deea2 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -59,7 +59,7 @@ Future<Set> loadTransformers(AssetEnvironment environment,
       if (error.type != 'IsolateSpawnException') throw error;
       // TODO(nweiz): don't parse this as a string once issues 12617 and 12689
       // are fixed.
-      if (!error.message.split('\n')[1].endsWith("import '$uri';")) {
+      if (!error.message.split('\n')[1].startsWith("Failure getting $uri:")) {
         throw error;
       }