Add new core ("dart:") libraries to frog so that apidoc can find them.

utils/apidoc/apidoc.dart

Index: utils/apidoc/apidoc.dart
diff --git a/utils/apidoc/apidoc.dart b/utils/apidoc/apidoc.dart
index 1ed08976a81b199ebf3524e486f02169a114bc38..f1b385dba0d1fd38515e661e8e5fff19637be498 100644
--- a/utils/apidoc/apidoc.dart
+++ b/utils/apidoc/apidoc.dart
@@ -35,7 +35,7 @@ void main() {
   // TODO(rnystrom): Cram in the the IO libraries. This is hackish right now.
   // We should probably move corelib stuff completely out of dartdoc and have it
   // all here.
-  world.getOrAddLibrary('io_libs.dart');
+  world.getOrAddLibrary('dart:io');
 
   print('Generating docs...');
   final apidoc = new Apidoc(mdn);