Support the new isolate load error messages.

Closes #1387

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1668983003 .
diff --git a/lib/src/barback/transformer_isolate.dart b/lib/src/barback/transformer_isolate.dart
index e833be0..ae025c8 100644
--- a/lib/src/barback/transformer_isolate.dart
+++ b/lib/src/barback/transformer_isolate.dart
@@ -91,9 +91,7 @@
         // "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(
-                "Load Error for") &&
-            firstErrorLine.contains(idsToUrls[id].path),
+            firstErrorLine.startsWith('Could not import "${idsToUrls[id]}"'),
             orElse: () => throw error);
         var packageUri = idToPackageUri(idsToAssetIds[missingTransformer]);
 
diff --git a/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart b/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
index f2c6e6a..2a30722 100644
--- a/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
+++ b/test/transformer/fails_to_load_a_transform_with_an_import_error_test.dart
@@ -28,8 +28,7 @@
     pubGet();
     var pub = startPubServe();
     pub.stderr.expect("Unable to spawn isolate: Unhandled exception:");
-    pub.stderr.expect(
-        startsWith("Load Error for "));
+    pub.stderr.expect(startsWith('Could not import "'));
     pub.shouldExit(1);
   });
 }