Add a draft of traversing visitor which will analyse function.

lib/compiler/implementation/compiler.dart

 // Copyright (c) 2012, 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.
 
 class WorkItem {
   final Element element;
   TreeElements resolutionTree;
   bool allowSpeculativeOptimization = true;
   List<HTypeGuard> guards = const <HTypeGuard>[];
 
@@ skipping 53 matching lines @@
 
   void processNativeClasses(world, libraries) {
     native.processNativeClasses(world, emitter, libraries);
   }
 
   void assembleProgram() {
     emitter.assembleProgram();
   }
 }
 
-class DartBackend extends Backend {
-  DartBackend(Compiler compiler) : super(compiler);
-  final List<CompilerTask> tasks = const <CompilerTask>[];
-}
-
 class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
   World world;
   String assembledCode;
   Namer namer;
   Types types;
   final bool enableTypeAssertions;
 
   final Tracer tracer;
@@ skipping 67 matching lines @@
     namer = new Namer(this);
     constantHandler = new ConstantHandler(this);
     scanner = new ScannerTask(this);
     dietParser = new DietParserTask(this);
     parser = new ParserTask(this);
     validator = new TreeValidatorTask(this);
     unparseValidator = new UnparseValidator(this, validateUnparse);
     resolver = new ResolverTask(this);
     checker = new TypeCheckerTask(this);
     backend = emitJavascript ?
-        new JavaScriptBackend(this) : new DartBackend(this);
+        new JavaScriptBackend(this) : new dart_backend.DartBackend(this);
     enqueuer = new EnqueueTask(this);
     tasks = [scanner, dietParser, parser, resolver, checker,
              unparseValidator, constantHandler, enqueuer];
     tasks.addAll(backend.tasks);
   }
 
   Universe get codegenWorld() => enqueuer.codegen.universe;
 
   int getNextFreeClassId() => nextFreeClassId++;
 
@@ skipping 418 matching lines @@
     // invariant that endOffset > beginOffset, but for EOF the
     // charoffset of the next token may be [beginOffset]. This can
     // also happen for synthetized tokens that are produced during
     // error handling.
     final endOffset =
       Math.max((end.next !== null) ? end.next.charOffset : 0, beginOffset + 1);
     assert(endOffset > beginOffset);
     return f(beginOffset, endOffset);
   }
 }