Recompute librariesByName after loading a deferred library.

tests/lib/mirrors/deferred_mirrors_update_test.dart

+library main;
+
+// Test that the library-mirrors are updated after loading a deferred library.
+
+@MirrorsUsed(targets: "D")
+import "dart:mirrors";
+
+import "deferred_mirrors_update_lib.dart" deferred as l;
+
+class D {}
+
+void main() {
+  print(reflectClass(D).owner);
+  l.loadLibrary().then((_) {
+    l.foo();
+  });
+}