anchor.diff:          reference.addEdit(change, '');
         } else {
           SimpleIdentifier interpolationIdentifier =
  -            await _getInterpolationIdentifier(reference);
  +            _getInterpolationIdentifier(reference);
           if (interpolationIdentifier != null) {
             doSourceChange_addElementEdit(
                 change,
  
positive.code:       PrefixElement prefix = element.prefix;
        SourceEdit edit = null;
        if (newName.isEmpty) {
          ImportDirective node = await _findNode();
          int uriEnd = node.uri.end;
          int prefixEnd = element.prefixOffset + prefix.nameLength;
          edit = newSourceEdit_range(
              range.startOffsetEndOffset(uriEnd, prefixEnd), "");
        } else {
          if (prefix == null) {
            ImportDirective node = await _findNode();
            int uriEnd = node.uri.end;
            edit =
                newSourceEdit_range(new SourceRange(uriEnd, 0), " as $newName");
  
negative.code:    * an [InterpolationExpression] without surrounding curly brackets, return it.
     * Otherwise return `null`.
     */
    SimpleIdentifier _getInterpolationIdentifier(SourceReference reference) {
      Source source = reference.element.source;
      CompilationUnit unit = session.getParsedUnit(source.fullName).unit;
      NodeLocator nodeLocator = new NodeLocator(reference.range.offset);
  