Fix warning after loader refactor discovered by a red bot.

pkg/compiler/lib/src/library_loader.dart

Index: pkg/compiler/lib/src/library_loader.dart
diff --git a/pkg/compiler/lib/src/library_loader.dart b/pkg/compiler/lib/src/library_loader.dart
index 48c404e664aa9de4a5f655d80f852c5fd8f88199..e6eaf88f8029627014f8eb6723ed7b5b79cc88dc 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -577,7 +577,7 @@ class _LibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
     return reporter.withCurrentElement(library, () {
       return scriptLoader.readScript(readableUri, part).then((Script script) {
         if (script == null) return;
-        return createUnitSync(script, library);
+        createUnitSync(script, library);
       });
     });
   }