Recompute librariesByName after loading a deferred library.

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index e56c3c839211457c7cbe2cf3ffe66fd962ab177a..9f90fa817d8165391dda4f4df3aa247a798d109f 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -3257,6 +3257,11 @@ LoadLibraryFunctionType _loadLibraryWrapper(String loadId) {
 final Map<String, Future<Null>> _loadingLibraries = <String, Future<Null>>{};
 final Set<String> _loadedLibraries = new Set<String>();
 
+typedef void DeferredLoadCallback();
+
+// Function that will be called every time a new deferred import is loaded.
+DeferredLoadCallback deferredLoadHook;
+
 Future<Null> loadDeferredLibrary(String loadId) {
   // For each loadId there is a list of hunk-uris to load, and a corresponding
   // list of hashes. These are stored in the app-global scope.
@@ -3277,7 +3282,10 @@ Future<Null> loadDeferredLibrary(String loadId) {
       // TODO(floitsch): Replace unsafe access to embedded global.
       JS('void', 'init.initializeLoadedHunk(#)', hash);
     }
-    _loadedLibraries.add(loadId);
+    bool updated = _loadedLibraries.add(loadId);
+    if (updated && deferredLoadHook != null) {
+      deferredLoadHook();
+    }
   });
 }