fix #27672, call computeConstantValue for metadata

pkg/dev_compiler/lib/src/compiler/element_helpers.dart

Index: pkg/dev_compiler/lib/src/compiler/element_helpers.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/element_helpers.dart b/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
index 053227dfa8719f607e3946095dd487d67b9d467a..7f4576d9252639a15a2f5da7710b222de5b6a41c 100644
--- a/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
+++ b/pkg/dev_compiler/lib/src/compiler/element_helpers.dart
@@ -51,7 +51,7 @@ class Tuple2<T0, T1> {
 ///
 DartObject findAnnotation(Element element, bool test(DartObjectImpl value)) {
   for (var metadata in element.metadata) {
-    var value = metadata.constantValue;
+    var value = metadata.computeConstantValue();
     if (value != null && test(value)) return value;
   }
   return null;