Load builtin helper libraries on demand.

sdk/lib/_internal/compiler/implementation/library_loader.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 dart2js;
 
 /**
  * [CompilerTask] for loading libraries and setting up the import/export scopes.
  *
  * The library loader uses four different kinds of URIs in different parts of
@@ skipping 202 matching lines @@
 }
 
 /**
  * Implementation class for [LibraryLoader]. The distinction between
  * [LibraryLoader] and [LibraryLoaderTask] is made to hide internal members from
  * the [LibraryLoader] interface.
  */
 class LibraryLoaderTask extends LibraryLoader {
   LibraryLoaderTask(Compiler compiler) : super(compiler);
   String get name => 'LibraryLoader';
+  List onLibraryLoadedCallbacks = [];
 
   final Map<String, LibraryElement> libraryNames =
       new LinkedHashMap<String, LibraryElement>();
 
   LibraryDependencyHandler currentHandler;
 
   LibraryElement loadLibrary(Uri resolvedUri, Node node, Uri canonicalUri) {
     return measure(() {
       assert(currentHandler == null);
       currentHandler = new LibraryDependencyHandler(compiler);
       LibraryElement library =
           createLibrary(currentHandler, null, resolvedUri, node, canonicalUri);
       currentHandler.computeExports();
       currentHandler = null;
+      var workList = onLibraryLoadedCallbacks;
+      onLibraryLoadedCallbacks = [];
+      workList.forEach((f) => f());
       return library;
     });
   }
 
   /**
    * Processes the library tags in [library].
    *
    * The imported/exported libraries are loaded and processed recursively but
    * the import/export scopes are not set up.
    */
@@ skipping 179 matching lines @@
       library = createLibrary();
     } else {
       library = compiler.libraries.putIfAbsent(canonicalUri.toString(),
                                                createLibrary);
     }
     if (newLibrary) {
       compiler.withCurrentElement(library, () {
         compiler.scanner.scanLibrary(library);
         processLibraryTags(handler, library);
         handler.registerLibraryExports(library);
-        compiler.onLibraryScanned(library, resolvedUri);
+        onLibraryLoadedCallbacks.add(
+            () => compiler.onLibraryLoaded(library, resolvedUri));
       });
     }
     return library;
   }
 
   // TODO(johnniwinther): Remove this method when 'js_helper' is handled by
   // [LibraryLoaderTask].
   void importLibrary(LibraryElement importingLibrary,
                      LibraryElement importedLibrary,
                      Import tag) {
@@ skipping 382 matching lines @@
   }
 
   /**
    * Registers all top-level entities of [library] as starting point for the
    * fixed-point computation of the import/export scopes.
    */
   void registerLibraryExports(LibraryElement library) {
     nodeMap[library].registerInitialExports();
   }
 }