Make the constant member value final

lib/constant_member.dart

diff --git a/lib/constant_member.dart b/lib/constant_member.dart
index 09490d1..db8c7ed 100644
--- a/lib/constant_member.dart
+++ b/lib/constant_member.dart
@@ -5,7 +5,7 @@
 part of cassowary;
 
 class ConstantMember extends _EquationMember {
-  double value = 0.0;
+  final double value;
 
   bool get isConstant => true;