Add mirrors tests

test/codegen/lib/mirrors/deferred_mirrors_metadata_lib.dart

+library lib;
+
+import "deferred_mirrors_metadata_test.dart";
+@MirrorsUsed(
+    targets: const ["main", "main.A", "main.B", "main.C", "main.D", "lib.E"])
+import "dart:mirrors";
+
+class H {
+  const H();
+}
+
+class F {
+  @H() final int f;
+}
+
+@C() class E {
+  @D() var f;
+}
+
+String foo() {
+  String c = reflectClass(E).metadata[0].invoke(#toString, []).reflectee;
+  String d = reflectClass(E).declarations[#f].metadata[0]
+      .invoke(#toString, []).reflectee;
+  InstanceMirror i = currentMirrorSystem().findLibrary(#main).metadata[0];
+  String a = i.invoke(#toString, []).reflectee;
+  String b = i.getField(#b).invoke(#toString, []).reflectee;
+  return a + b + c + d;
+}
+