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..9edfdcef53a32b1e47915b4569970f9cbc962424 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 of `uri`, and call
+//    successCallback. If it fails to do so, it should call errorCallback with
+//    an error.
 """;
 
 // Compact field specifications.  The format of the field specification is