Support for a dart2js `dartDeferredLoader` hook.

pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart
index 4c2a9229943590025782dad0a2722d2b2ea394d3..986236368b716046a9a15f695d0996d22c2e4a8b 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/declarations.dart
@@ -57,6 +57,12 @@ const String HOOKS_API_USAGE = """
 //    if this function is defined, the Dart [main] method will not be invoked
 //    directly. Instead, a closure that will invoke [main], and its arguments
 //    [args] is passed to [dartMainRunner].
+//
+// dartDeferredLibraryLoader(uri, successCallback, errorCallback):
+//    if this function is defined, it will be called when a deferered library
+//    is loaded. It should load and eval the javascript on uri, and call

[Lasse Reichstein Nielsen, 2015/02/26 08:44:09]

javascript on uri -> javascript of `uri`

[sigurdm, 2015/02/26 13:19:05]

Done.

+//    successCallback. If it fails to do so, it should call errorCallback with
+//    an error.
 """;
 
 // Compact field specifications.  The format of the field specification is