First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, 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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 487 matching lines @@
         nonPlatformSize += compilationUnit.script.text.length;
       }
     }
     int percentage = compiler.assembledCode.length * 100 ~/ nonPlatformSize;
     log('Total used non-platform files size: ${nonPlatformSize} bytes, '
         'bundle size: ${compiler.assembledCode.length} bytes (${percentage}%)');
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 
-  void onLibraryLoaded(LibraryElement library, Uri uri) {
+  Future onLibraryLoaded(LibraryElement library, Uri uri) {
     if (useMirrorHelperLibrary && library == compiler.mirrorsLibrary) {
-      mirrorHelperLibrary = compiler.scanBuiltinLibrary(
-          MirrorRenamer.MIRROR_HELPER_LIBRARY_NAME);
-      mirrorHelperGetNameFunction = mirrorHelperLibrary.find(
-          const SourceString(MirrorRenamer.MIRROR_HELPER_GET_NAME_FUNCTION));
-      mirrorHelperSymbolsMap = mirrorHelperLibrary.find(
-          const SourceString(MirrorRenamer.MIRROR_HELPER_SYMBOLS_MAP_NAME));
+      return compiler.scanBuiltinLibrary(
+          MirrorRenamer.MIRROR_HELPER_LIBRARY_NAME).
+          then((LibraryElement element) {
+        mirrorHelperLibrary = element;
+        mirrorHelperGetNameFunction = mirrorHelperLibrary.find(
+            const SourceString(MirrorRenamer.MIRROR_HELPER_GET_NAME_FUNCTION));
+        mirrorHelperSymbolsMap = mirrorHelperLibrary.find(
+            const SourceString(MirrorRenamer.MIRROR_HELPER_SYMBOLS_MAP_NAME));
+      });
     }
+    return new Future.value();
   }
 
   void registerStaticSend(Element element, Node node) {
     if (useMirrorHelperLibrary) {
       mirrorRenamer.registerStaticSend(element, node);
     }
   }
 
   void registerMirrorHelperElement(Element element, Node node) {
     if (mirrorHelperLibrary != null
@@ skipping 86 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().canonicalUri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Iterable<Element> elements) =>
     sorted(elements, compareElements);