First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/deferred_load.dart

Index: sdk/lib/_internal/compiler/implementation/deferred_load.dart
diff --git a/sdk/lib/_internal/compiler/implementation/deferred_load.dart b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
index 53eca9743ed7c90cbebcf36cd64e57b6d68b5c56..4b6154846844d5b1b2d3e3f0e99206ed38aaf91e 100644
--- a/sdk/lib/_internal/compiler/implementation/deferred_load.dart
+++ b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
@@ -4,6 +4,7 @@
 
 library deferred_load;
 
+import 'dart:async';
 import 'dart:collection'
        show LinkedHashMap,
             LinkedHashSet;
@@ -47,8 +48,6 @@ class DeferredLoadTask extends CompilerTask {
   /// should become obsolete.
   final Set<Element> allDeferredElements = new LinkedHashSet<Element>();
 
-  ClassElement cachedDeferredLibraryClass;
-
   DeferredLoadTask(Compiler compiler) : super(compiler);
 
   String get name => 'Deferred Loading';