Don't use "var" to introduce the deferred_initializer.

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
     CLASS_FIELDS_EXTRACTOR,
@@ skipping 234 matching lines @@
     Map<DeferredFragment, String> hunkHashes = <DeferredFragment, String>{};
 
     fragmentsCode.forEach((DeferredFragment fragment, js.Expression code) {
       hunkHashes[fragment] = writeDeferredFragment(fragment, code);
     });
 
     return hunkHashes;
   }
 
   js.Statement buildDeferredInitializerGlobal() {
-    String global = deferredInitializersGlobal;
-    return js.js.statement(
-        "if (typeof($global) === 'undefined') var # = Object.create(null);",
-        new js.VariableDeclaration(global, allowRename: false));
+    return js.js.statement('self.#deferredInitializers = '
+        'self.#deferredInitializers || Object.create(null);',
+        {'deferredInitializers': deferredInitializersGlobal});
   }
 
   // Writes the given [fragment]'s [code] into a file.
   //
   // Updates the shared [outputBuffers] field with the output.
   void writeMainFragment(MainFragment fragment, js.Statement code,
       {bool isSplit}) {
     LineColumnCollector lineColumnCollector;
     List<CodeOutputListener> codeOutputListeners;
     if (shouldGenerateSourceMap) {
@@ skipping 146 matching lines @@
     // Json does not support comments, so we embed the explanation in the
     // 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.deferredMapUri.path, 'deferred_map')
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }