Assert that we don't mix K and J elements

pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart

 // Copyright (c) 2015, 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.js_emitter.startup_emitter.model_emitter;
 
 import 'dart:convert' show JsonEncoder;
 import 'dart:math' show Random;
 
 import 'package:js_runtime/shared/embedded_names.dart'
@@ skipping 67 matching lines @@
   static const String deferredExtension = "part.js";
 
   static const String typeNameProperty = r"builtin$cls";
 
   ModelEmitter(this.compiler, this.namer, this.nativeEmitter, this._closedWorld,
       CodeEmitterTask task, this.shouldGenerateSourceMap) {
     this.constantEmitter = new ConstantEmitter(
         compiler.options,
         _closedWorld.commonElements,
         compiler.codegenWorldBuilder,
-        compiler.backend.rtiNeed,
+        _closedWorld.rtiNeed,
         compiler.backend.rtiEncoder,
         namer,
         task,
         this.generateConstantReference,
         constantListGenerator);
   }
 
   DiagnosticReporter get reporter => compiler.reporter;
 
   js.Expression constantListGenerator(js.Expression array) {
@@ skipping 296 matching lines @@
     // data.
     mapping["_comment"] = "This mapping shows which compiled `.js` files are "
         "needed for a given deferred library import.";
     mapping.addAll(compiler.deferredLoadTask.computeDeferredMap());
     compiler.outputProvider(
         compiler.options.deferredMapUri.path, '', OutputType.info)
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }