dart2js: allow empty main fragment in emitter.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

 // Copyright (c) 2014, 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.
 
 part of dart2js.js_emitter;
 
 
 class OldEmitter implements Emitter {
   final Compiler compiler;
   final CodeEmitterTask task;
@@ skipping 1252 matching lines @@
     mainOutput.add('$isolateProperties$_=$_$isolatePropertiesName$N');
 
     emitFunctionThatReturnsNull(mainOutput);
 
     Iterable<LibraryElement> libraries =
         task.outputLibraryLists[mainOutputUnit];
     if (libraries == null) libraries = [];
     emitMangledNames(mainOutput);
 
     Map<Element, ClassBuilder> descriptors = elementDescriptors[mainFragment];
+    if (descriptors == null) descriptors = const {};
+
     checkEverythingEmitted(descriptors.keys);
 
     CodeBuffer libraryBuffer = new CodeBuffer();
     for (LibraryElement library in Elements.sortedByPosition(libraries)) {
       writeLibraryDescriptor(libraryBuffer, library, mainFragment);
       descriptors.remove(library);
     }
 
     if (descriptors.isNotEmpty) {
       List<Element> remainingLibraries = descriptors.keys
@@ skipping 611 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }