Handle use of constant constructors with default values.

pkg/compiler/lib/src/serialization/modelz.dart

Index: pkg/compiler/lib/src/serialization/modelz.dart
diff --git a/pkg/compiler/lib/src/serialization/modelz.dart b/pkg/compiler/lib/src/serialization/modelz.dart
index ac8c8763f4c1fb21529da1b12a92f62e141af078..f721eb17d400258bff929f4cc35f82eb832f9cb1 100644
--- a/pkg/compiler/lib/src/serialization/modelz.dart
+++ b/pkg/compiler/lib/src/serialization/modelz.dart
@@ -1411,6 +1411,9 @@ abstract class FieldElementZ extends DeserializedElementZ
   }
 
   @override
+  bool get hasConstant => true;
+
+  @override
   ConstantExpression get constant {
     _ensureConstant();
     return _constant;
@@ -1858,6 +1861,9 @@ abstract class ParameterElementZ extends DeserializedElementZ
   bool get isConst => false;
 
   @override
+  bool get hasConstant => true;
+
+  @override
   ConstantExpression get constant {
     if (isOptional) {
       if (_constant == null) {
@@ -1986,6 +1992,9 @@ class LocalVariableElementZ extends DeserializedElementZ
   }
 
   @override
+  bool get hasConstant => true;
+
+  @override
   ConstantExpression get constant {
     if (isConst) {
       return _constant;