anchor.diff:  
       if (path.length < 4) return null;
   
  +    if (_pkgPath == null) {
  +      File packageFile = new File('.packages');
  +
  +      if (packageFile.existsSync()) {
  +        List<String> lines = packageFile.readAsLinesSync();
  +        String compilerLn =
  +          lines.firstWhere((ln) => ln.startsWith("compiler_unsupported:"));
  +        compilerLn =
  +            compilerLn.substring("compiler_unsupported:".length) + 'sdk';
  +        if (compilerLn.startsWith("file://")) compilerLn = compilerLn.substring(("file://").length);
  +        _pkgPath = compilerLn;
  +      }
  +    }
       // Remove the `/lib` dir.
       var p = _pkgPath + path.substring(4) + '_';
   
  
positive.code: /**
   * This `DartSdk` implementation reads the SDK resources encoded into the
   * `compiler_unsupported` package as part of a build step. This does not rely
   * on being able to locate an SDK on disk. It _does_ need to have a populated
   * packages directory in the current working directory.
   */
  class DartSdkIO implements DartSdk {
    static final _pkgPath = 'packages/compiler_unsupported/sdk';
  
    Map<String, String> _cache = {};
    ZLibCodec _zlib = new ZLibCodec();
  
negative.code: // 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.
  
  