anchor.diff:    }
   
     Future<T> _runWithFrontEndContext<T>(String msg, Future<T> f()) async {
  -    // TODO(brianwilkerson) Determine whether this await is necessary.
  -    await null;
       return await CompilerContext.runWithOptions(_options, (context) {
         context.disableColors();
         return _logger.runAsync(msg, f);
  
positive.code:       _component.libraries.remove(library);
        _component.root.removeChild('${library.importUri}');
        _component.uriToSource.remove(libraryUri);
        _results.remove(library.importUri);
  
        // Recursively invalidate dependencies.
  
negative.code:     }
  
      return _runWithFrontEndContext('Compile', () async {
        try {
          // Initialize the dill target once.
          if (_dillTarget == null) {
            _dillTarget =
                new DillTarget(_options.ticker, uriTranslator, _options.target);
          }
  
          // Append new libraries from the current component.
          await _logger.runAsync('Load dill libraries', () async {
            _dillTarget.loader.appendLibraries(_component,
                filter: (uri) => !_dillTarget.loader.builders.containsKey(uri));
            await _dillTarget.buildOutlines();
          });
  
          // Create the target to compile the library.
          var kernelTarget = new _AnalyzerKernelTarget(_fileSystem, _dillTarget,
              uriTranslator, new AnalyzerMetadataCollector());
          kernelTarget.read(uri);
  
  