dart2js: The main output unit always has a descriptor (at the very least `main`).

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 1441 matching lines @@
     if (classes != null) {
       for (ClassElement element in classes) {
         generateClass(element, getElementDescriptor(element));
       }
     }
 
     if (compiler.enableMinification) {
       mainBuffer.write(';');
     }
 
-    if (elementDescriptors.isNotEmpty) {
-      Iterable<LibraryElement> libraries =
-          task.outputLibraryLists[mainOutputUnit];
-      if (libraries == null) libraries = [];
-      emitLibraries(libraries);
-      emitTypedefs();
-      emitMangledNames();
+    Iterable<LibraryElement> libraries =
+        task.outputLibraryLists[mainOutputUnit];
+    if (libraries == null) libraries = [];
+    emitLibraries(libraries);
+    emitTypedefs();
+    emitMangledNames();
 
-      checkEverythingEmitted(elementDescriptors.keys);
+    checkEverythingEmitted(elementDescriptors.keys);
 
-      CodeBuffer libraryBuffer = new CodeBuffer();
-      for (LibraryElement library in Elements.sortedByPosition(libraries)) {
-        writeLibraryDescriptors(libraryBuffer, library);
-        elementDescriptors.remove(library);
-      }
+    CodeBuffer libraryBuffer = new CodeBuffer();
+    for (LibraryElement library in Elements.sortedByPosition(libraries)) {
+      writeLibraryDescriptors(libraryBuffer, library);
+      elementDescriptors.remove(library);
+    }
 
-      mainBuffer
-          ..write(
-              jsAst.prettyPrint(
-                  getReflectionDataParser(this, backend),
-                  compiler))
-          ..write(n);
+    mainBuffer
+        ..write(
+            jsAst.prettyPrint(
+                getReflectionDataParser(this, backend),
+                compiler))
+        ..write(n);
 
-      mainBuffer..write('$parseReflectionDataName([$n')
-                ..write(libraryBuffer)
-                ..write('])$N');
-    }
+    mainBuffer..write('$parseReflectionDataName([$n')
+              ..write(libraryBuffer)
+              ..write('])$N');
 
     interceptorEmitter.emitGetInterceptorMethods(mainBuffer);
     interceptorEmitter.emitOneShotInterceptors(mainBuffer);
 
     if (task.outputContainsConstantList) {
       emitMakeConstantList(mainBuffer);
     }
 
     // Constants in checked mode call into RTI code to set type information
     // which may need getInterceptor (and one-shot interceptor) methods, so
@@ skipping 534 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }