Recompute librariesByName after loading a deferred library.

tests/lib/mirrors/deferred_mirrors_update_lib.dart

Index: tests/lib/mirrors/deferred_mirrors_update_lib.dart
diff --git a/tests/lib/mirrors/deferred_mirrors_update_lib.dart b/tests/lib/mirrors/deferred_mirrors_update_lib.dart
new file mode 100644
index 0000000000000000000000000000000000000000..786831523ac6c67d71cc55a686c6f6ef353afeb5
--- /dev/null
+++ b/tests/lib/mirrors/deferred_mirrors_update_lib.dart
@@ -0,0 +1,11 @@
+library lib;
+
+@MirrorsUsed(targets: "lib.C")
+import "dart:mirrors";
+
+class C {}
+
+foo() {
+  var a = new C();
+  print(reflectClass(C).owner);
+}