Split backend implementation of EnqueuerListener

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 'package:js_runtime/shared/embedded_names.dart'
     show
@@ skipping 176 matching lines @@
     // Now that we have written the deferred hunks, we can update the hash
     // tokens in the main-fragment.
     deferredHashTokens
         .forEach((DeferredFragment key, _DeferredFragmentHash token) {
       token.setHash(hunkHashes[key]);
     });
 
     writeMainFragment(mainFragment, mainCode,
         isSplit: program.deferredFragments.isNotEmpty);
 
-    if (backend.requiresPreamble && !backend.htmlLibraryIsLoaded) {
+    if (backend.backendUsage.requiresPreamble && !backend.htmlLibraryIsLoaded) {
       reporter.reportHintMessage(NO_LOCATION_SPANNABLE, MessageKind.PREAMBLE);
     }
 
     if (compiler.options.deferredMapUri != null) {
       writeDeferredMap();
     }
 
     // Return the total program size.
     return outputBuffers.values.fold(0, (a, b) => a + b.length);
   }
@@ skipping 188 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, 'deferred_map')
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }