Correctly resolve package parts.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.source_library_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 113 matching lines @@
     imports.add(new Import(loader.read(resolve(uri)), prefix, combinators));
   }
 
   void addPart(List<MetadataBuilder> metadata, String path) {
     Uri resolvedUri;
     Uri newFileUri;
     if (uri.scheme == "dart") {
       resolvedUri = new Uri(scheme: "dart", path: "${uri.path}/$path");
       newFileUri = fileUri.resolve(path);
     } else {
-      newFileUri = resolvedUri = resolve(path);
+      resolvedUri = uri.resolve(path);
+      newFileUri = fileUri.resolve(path);
     }
     parts.add(loader.read(resolvedUri, newFileUri));
   }
 
   void addPartOf(List<MetadataBuilder> metadata, String name) {
     partOf = name;
   }
 
   ClassBuilder addClass(List<MetadataBuilder> metadata,
       int modifiers, String name,
@@ skipping 188 matching lines @@
   }
 
   int resolveConstructors(_) {
     int count = 0;
     members.forEach((String name, Builder member) {
       count += member.resolveConstructors(this);
     });
     return count;
   }
 }