Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/parser/parser_task.dart

 // Copyright (c) 2011, 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.
 
 library dart2js.parser.task;
 
 import '../common.dart';
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../elements/modelx.dart' show ElementX;
@@ skipping 18 matching lines @@
 
   Node parseCompilationUnit(Token token) {
     return measure(() {
       NodeListener listener =
           new NodeListener(const ScannerOptions(), compiler.reporter, null);
       Parser parser = new Parser(listener);
       try {
         parser.parseUnit(token);
       } on ParserError catch (_) {
         assert(invariant(compiler.reporter.spanFromToken(token),
-                compiler.compilationFailed));
+            compiler.compilationFailed));
         return listener.makeNodeList(0, null, null, '\n');
       }
       Node result = listener.popNode();
       assert(listener.nodes.isEmpty);
       return result;
     });
   }
 }