Start moving the JavaScript backend related code into a separate library

lib/compiler/implementation/native_handler.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.
 
 #library('native');
 #import('dart:uri');
 #import('leg.dart');
 #import('elements/elements.dart');
+#import('js_backend/js_backend.dart');
 #import('scanner/scannerlib.dart');
 #import('ssa/ssa.dart');
 #import('tree/tree.dart');
 #import('util/util.dart');
 
 void processNativeClasses(Enqueuer world,
                           CodeEmitterTask emitter,
                           Collection<LibraryElement> libraries) {
   for (LibraryElement library in libraries) {
     processNativeClassesInLibrary(world, emitter, library);
@@ skipping 310 matching lines @@
                                       String parameters) {
   buffer.add("  if (Object.getPrototypeOf(this).hasOwnProperty");
   buffer.add("('$methodName')) {\n");
   buffer.add("  $code");
   buffer.add("  } else {\n");
   buffer.add("    return Object.prototype.$methodName.call(this");
   buffer.add(parameters == '' ? '' : ', $parameters');
   buffer.add(");\n");
   buffer.add("  }\n");
 }