Handle use of constant constructors with default values.

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 4e0c2ef11f49a396f4d965fa60970c03201ab48d..657020647848e18a3100a71af44a52c9e12b6860 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -171,6 +171,9 @@ class ClosureFieldElement extends ElementX
   List<FunctionElement> get nestedClosures => const <FunctionElement>[];
 
   @override
+  bool get hasConstant => false;
+
+  @override
   ConstantExpression get constant => null;
 }
 
@@ -310,6 +313,9 @@ class BoxFieldElement extends ElementX
   }
 
   @override
+  bool get hasConstant => false;
+
+  @override
   ConstantExpression get constant => null;
 }