Adding some asset-related utilities to code_transformers

pkg/code_transformers/lib/src/resolver_impl.dart

Index: pkg/code_transformers/lib/src/resolver_impl.dart
diff --git a/pkg/code_transformers/lib/src/resolver_impl.dart b/pkg/code_transformers/lib/src/resolver_impl.dart
index 9322ccd5f8af4f7d409342c36fff0b9141478acc..2a8f4631a30f7e2291ac852ac467c777f7a62b37 100644
--- a/pkg/code_transformers/lib/src/resolver_impl.dart
+++ b/pkg/code_transformers/lib/src/resolver_impl.dart
@@ -17,6 +17,7 @@ import 'package:path/path.dart' as native_path;
 import 'package:source_maps/refactor.dart';
 import 'package:source_maps/span.dart' show SourceFile, Span;
 
+import 'assets.dart';
 import 'resolver.dart';
 
 // We should always be using url paths here since it's always Dart/pub code.
@@ -509,14 +510,7 @@ AssetId _resolve(AssetId source, String url, TransformLogger logger,
   // Dart SDK libraries do not have assets.
   if (uri.scheme == 'dart') return null;
 
-  if (uri.host != '' || uri.scheme != '' || path.isAbsolute(url)) {
-    logger.error('absolute paths not allowed: "$url"', span: span);
-    return null;
-  }
-
-  var targetPath = path.normalize(
-      path.join(path.dirname(source.path), url));
-  return new AssetId(source.package, targetPath);
+  return uriToAssetId(source, url, logger, span);
 }