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

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

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index a69fc80565b7fa30472c8a07e3c96139606f889e..66d92ee7e8ea3311edcdc30481d07ad7c2e9d746 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -1459,33 +1459,31 @@ class OldEmitter implements Emitter {
       mainBuffer.write(';');
     }
 
-    if (elementDescriptors.isNotEmpty) {
-      Iterable<LibraryElement> libraries =
-          task.outputLibraryLists[mainOutputUnit];
-      if (libraries == null) libraries = [];
-      emitLibraries(libraries);
-      emitTypedefs();
-      emitMangledNames();
-
-      checkEverythingEmitted(elementDescriptors.keys);
-
-      CodeBuffer libraryBuffer = new CodeBuffer();
-      for (LibraryElement library in Elements.sortedByPosition(libraries)) {
-        writeLibraryDescriptors(libraryBuffer, library);
-        elementDescriptors.remove(library);
-      }
+    Iterable<LibraryElement> libraries =
+        task.outputLibraryLists[mainOutputUnit];
+    if (libraries == null) libraries = [];
+    emitLibraries(libraries);
+    emitTypedefs();
+    emitMangledNames();
+
+    checkEverythingEmitted(elementDescriptors.keys);
+
+    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);