anchor.diff:  import 'package:analyzer/src/generated/java_io.dart' show JavaFile;
   import 'package:analyzer/src/generated/source_io.dart' show FileBasedSource;
   import 'package:path/path.dart' as pathos;
  -
  -class _SdkExtFileBasedSource extends FileBasedSource {
  -  _SdkExtFileBasedSource(JavaFile file, Uri uri)
  -      : super(file, uri);
  -}
   
   /// Given a packageMap (see [PackageMapProvider]), check in each package's lib
   /// directory for the existence of a `.sdkext` file. This file must contain a
  
positive.code: 
    @override
    Uri restoreAbsolute(Source source) {
      if (source is _SdkExtFileBasedSource) {
        return source.uri;
      }
      return null;
  
negative.code:     var partUri = new Uri.file(pathos.join(directory, partPath));
      assert(partUri.isAbsolute);
      JavaFile javaFile = new JavaFile.fromUri(partUri);
      return new FileBasedSource(javaFile, importUri);
    }
  
    /// Resolve an import of an sdk extension.
    Source _resolveEntry(Uri libraryEntry, Uri importUri) {
      // Library entry.
      JavaFile javaFile = new JavaFile.fromUri(libraryEntry);
      return new FileBasedSource(javaFile, importUri);
    }
  
    /// Return the library name of [importUri].
    String _libraryName(Uri importUri) {
      var uri = importUri.toString();
      int index = uri.indexOf('/');
      if (index >= 0) {
        return uri.substring(0, index);
      }
      return uri;
    }
  
    /// Return the part path of [importUri].
    String _partPath(Uri importUri) {
      var uri = importUri.toString();
      int index = uri.indexOf('/');
      if (index >= 0) {
        return uri.substring(index + 1);
      }
      return null;
    }
  
    /// Returns true if [libraryName] is a registered sdk extension.
    bool _registeredSdkExtension(String libraryName) {
      return _urlMappings[libraryName] != null;
    }
  
    @override
    Source resolveAbsolute(Uri importUri) {
      String libraryName = _libraryName(importUri);
      String partPath = _partPath(importUri);
      // Lookup library name in mappings.
      String mapping = _urlMappings[libraryName];
      if (mapping == null) {
  