Parameter contexts are non-final

packages/cassowary/lib/param.dart

@@ -6,11 +6,9 @@ part of cassowary;
 
 class Param extends _EquationMember {
   final Variable variable;
-  final dynamic context;
+  dynamic context;
 
-  Param([double value = 0.0])
-      : variable = new Variable(value),
-        context = null {
+  Param([double value = 0.0]) : variable = new Variable(value) {
     variable._owner = this;
   }
 

packages/cassowary/test/cassowary_test.dart

@@ -579,4 +579,10 @@ void main() {
     expect(mid.value, 50.0);
     expect(right.value, 90.0);
   });
+
+  test('param_context_non_final', () {
+    var p = new Param.withContext("a");
+    p.context = "b";
+    expect(p.context, "b");
+  });
 }