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 46 matching lines @@
     return generator.generateMethod(work, graph);
   }
 
   void processNativeClasses(world, libraries) {
     native.processNativeClasses(world, emitter, libraries);
   }
 
   void assembleProgram() => emitter.assembleProgram();
 }
 
-class DartBackend extends Backend {
-  DartBackend(Compiler compiler) : super(compiler);
-}
-
 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 493 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);
   }
 }