dart2js: retain constants that are used as annotations in constructors and setters.

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

Index: pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
diff --git a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
index 3572ff417d7d49622697c6f5046a633eea5acfa9..dc2236886dbd2544b864c1d693e197cd124ab78f 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -196,7 +196,9 @@ class CodeEmitterTask extends CompilerTask {
       for (Element element in backend.generatedCode.keys) {
         if (backend.isAccessibleByReflection(element)) {
           bool shouldRetainMetadata = backend.retainMetadataOf(element);
-          if (shouldRetainMetadata && element.isFunction) {
+          if (shouldRetainMetadata &&
+              (element.isFunction || element.isConstructor ||
+               element.isSetter)) {
             FunctionElement function = element;
             function.functionSignature.forEachParameter(
                 backend.retainMetadataOf);