Add a draft of traversing visitor which will analyse function.

lib/compiler/implementation/leg.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('leg');
 
 #import('dart:uri');
 
 #import('colors.dart');
+#import('dart_backend/dart_backend.dart', prefix: 'dart_backend');
 #import('elements/elements.dart');
 #import('native_handler.dart', prefix: 'native');
 #import('scanner/scanner_implementation.dart');
 #import('scanner/scannerlib.dart');
 #import('ssa/ssa.dart');
 #import('string_validator.dart');
 #import('source_file.dart');
 #import('tree/tree.dart');
 #import('util/characters.dart');
 #import('util/util.dart');
 
 #source('compile_time_constants.dart');
 #source('compiler.dart');
 #source('diagnostic_listener.dart');
 #source('emitter.dart');
 #source('enqueue.dart');
 #source('namer.dart');
 #source('native_emitter.dart');
 #source('operations.dart');
 #source('resolver.dart');
 #source('script.dart');
 #source('tree_validator.dart');
 #source('typechecker.dart');
 #source('universe.dart');
 #source('unparse_validator.dart');
 #source('warnings.dart');
 #source('world.dart');