Make deferred loading and meta-data play better.

tests/lib/mirrors/deferred_mirrors_metadata_test.dart

+@A(const B())
+library main;
+
+@B()
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+
+import "dart:async";
+import "dart:math";
+
+@l import 'deferred_mirrors_metadata_lib.dart' as lib1;
+
+const l = const DeferredLibrary("lib1");
+
+
+class A {
+  final B b;
+  const A(this.b);
+  String toString() => "A";
+}
+
+class B {
+  const B();
+  String toString() => "B";
+}
+
+class C {
+  const C();
+  String toString() => "C";
+}
+
+void main() {
+  asyncStart();
+  l.load().then((_) {
+    Expect.equals("ABC", lib1.foo());
+    asyncEnd();
+  });
+}