anchor.diff:  // 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:collection';
   import 'dart:core';
   
   import 'package:analyzer/file_system/file_system.dart';
  
positive.code:   @override
    Source resolveAbsolute(Uri _ignore, [Uri uri]) {
      uri ??= _ignore;
      return _sourceCache.putIfAbsent(uri, () {
        if (uri.scheme != 'package') {
          return null;
        }
  
        Source basicResolverSource = _normalUriResolver.resolveAbsolute(uri);
        if (basicResolverSource != null && basicResolverSource.exists()) {
          return basicResolverSource;
        }
  
        String uriPath = uri.path;
        int slash = uriPath.indexOf('/');
  
        // If the path either starts with a slash or has no slash, it is invalid.
        if (slash < 1) {
          return null;
        }
  
        String packageName = uriPath.substring(0, slash);
        String fileUriPart = uriPath.substring(slash + 1);
        String filePath = fileUriPart.replaceAll('/', _context.separator);
  
        File file = _workspace.builtFile(
            _workspace.builtPackageSourcePath(filePath), packageName);
        if (file != null && file.exists) {
          return file.createSource(uri);
        }
        return basicResolverSource;
      });
    }
  
    @override
  
negative.code:   final UriResolver _normalUriResolver;
    final path.Context _context;
  
    PackageBuildPackageUriResolver(
        PackageBuildWorkspace workspace, this._normalUriResolver)
        : _workspace = workspace,
  