Support the new isolate load error messages.

lib/src/barback/transformer_isolate.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 
 import 'package:barback/barback.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 73 matching lines @@
         if (error.type != 'IsolateSpawnException') throw error;
 
         // TODO(nweiz): don't parse this as a string once issues 12617 and 12689
         // are fixed.
         var firstErrorLine = error.message.split('\n')[1];
 
         // The isolate error message contains the fully expanded path, not the
         // "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]);
 
         // If there was an IsolateSpawnException and the import that actually
         // failed was the one we were loading transformers from, throw an
         // application exception with a more user-friendly message.
         fail('Transformer library "$packageUri" not found.',
             error, stackTrace);
       });
     });
@@ skipping 26 matching lines @@
 /// An error thrown when a transformer fails to load.
 class TransformerLoadError extends SourceSpanException
     implements WrappedException {
   final CrossIsolateException innerError;
   Chain get innerChain => innerError.stackTrace;
 
   TransformerLoadError(CrossIsolateException error, SourceSpan span)
       : innerError = error,
         super("Error loading transformer: ${error.message}", span);
 }