anchor.diff:    final bool isIdentifier;
   
     SourceMapSpan(SourceLocation start, SourceLocation end, String text,
  -          {this.isIdentifier: false})
  +      {this.isIdentifier: false})
         : super(start, end, text);
   
     /// Creates a [SourceMapSpan] for an identifier with value [text] starting at
  
positive.code:   SourceMapFileSpan(this._inner, {this.isIdentifier: false});
  
    int compareTo(SourceSpan other) => _inner.compareTo(other);
    SourceSpan union(SourceSpan other) => _inner.union(other);
    FileSpan expand(FileSpan other) => _inner.expand(other);
    String message(String message, {color}) =>
        _inner.message(message, color: color);
    String toString() => _inner.toString()
        .replaceAll("FileSpan", "SourceMapFileSpan");
  }
  
negative.code:   /// The [end] location is determined by adding [text] to [start].
    SourceMapSpan.identifier(SourceLocation start, String text)
        : this(
              start,
              new SourceLocation(start.offset + text.length,
                  sourceUrl: start.sourceUrl,
                  line: start.line,
                  column: start.column + text.length),
              text,
              isIdentifier: true);
  }
  
  /// A wrapper aruond a [FileSpan] that implements [SourceMapSpan].
  