Copy compiled constants from metadataHandler to constantHandler.

dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 46abf0a923fb8f55543f97b7b95b2dcc45521abb..c8fd503630bb7be0186d4cab905d93266460ecdf 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -1403,6 +1403,13 @@ class JavaScriptBackend extends Backend {
   bool retainName(SourceString name) => mustPreserveNames;
 
   bool retainMetadataOf(Element element) {
+    if (mustRetainMetadata) {
+      // TODO(ahe): This is a little hacky, but I'll have to rewrite this when
+      // implementing @MirrorsUsed anyways.
+      compiler.constantHandler.compiledConstants.addAll(
+          compiler.metadataHandler.compiledConstants);
+      compiler.metadataHandler.compiledConstants.clear();
+    }
     if (mustRetainMetadata) hasRetainedMetadata = true;
     return mustRetainMetadata;
   }