Let deferred load work from worker isolates.

sdk/lib/async/deferred_load.dart

Index: sdk/lib/async/deferred_load.dart
diff --git a/sdk/lib/async/deferred_load.dart b/sdk/lib/async/deferred_load.dart
index 35ff034c634e3d771787f8ad51df07997bb2d2a2..43982ce58e14f866b794e533064f667b583634a9 100644
--- a/sdk/lib/async/deferred_load.dart
+++ b/sdk/lib/async/deferred_load.dart
@@ -36,6 +36,18 @@ class DeferredLibrary {
    *
    * The value of the returned future is true if this invocation of
    * [load] caused the library to be loaded.
+   *
+   * If the library fails to load, the Future will complete with a
+   * DeferredLoadException.
    */
   external Future<bool> load();
 }
+
+/**
+ * Thrown when a deferred library fails to load.
+ */
+class DeferredLoadException implements Exception {
+  DeferredLoadException(String this._s);
+  String toString() => "DeferredLoadException: '$_s'";
+  final String _s;
+}