Revert "Remove Registry from registerMetadataConstant" and "Serialize metadata"

tests/compiler/dart2js/mirrors_used_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Test that the @MirrorsUsed annotation suppress hints and that only
 /// requested elements are retained for reflection.
 library dart2js.test.mirrors_used_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 109 matching lines @@
                 compiler.backend.isAccessibleByReflection(classMember),
                 '$classMember');
           });
         } else {
           Expect.isFalse(
               compiler.backend.isAccessibleByReflection(member), '$member');
         }
       });
     }
 
-    int metadataCount = 0;
+    // There should at least be one metadata constant:
+    // 1. The constructed constant for 'MirrorsUsed'.
+    Expect.isTrue(backend.metadataConstants.length >= 1);
+
     Set<ConstantValue> compiledConstants = backend.constants.compiledConstants;
     // Make sure that most of the metadata constants aren't included in the
     // generated code.
-    backend.processMetadata(
-        compiler.enqueuer.resolution.processedElements, (metadata) {
-      ConstantValue constant =
-          backend.constants.getConstantValueForMetadata(metadata);
+    for (var dependency in backend.metadataConstants) {
+      ConstantValue constant = dependency.constant;
       Expect.isFalse(compiledConstants.contains(constant),
                      constant.toStructuredText());
-      metadataCount++;
-    });
-
-    // There should at least be one metadata constant:
-    // 1. The constructed constant for 'MirrorsUsed'.
-    Expect.isTrue(metadataCount >= 1);
+    }
 
     // The type literal 'Foo' is both used as metadata, and as a plain value in
     // the program. Make sure that it isn't duplicated.
     int fooConstantCount = 0;
     for (ConstantValue constant in compiledConstants) {
       if (constant is TypeConstantValue &&
           '${constant.representedType}' == 'Foo') {
         fooConstantCount++;
       }
     }
@@ skipping 29 matching lines @@
 library lib;
 
 import 'dart:mirrors';
 
 useReflect(type) {
   print(new Symbol('Foo'));
   print(MirrorSystem.getName(reflectClass(type).owner.qualifiedName));
 }
 """,
 };