force the contexts values type

dart/lib/src/scope.dart

@@ -57,7 +57,11 @@ class Scope {
 
   Contexts get contexts => _contexts;
 
-  void setContexts(String key, dynamic value) {
+  void setContexts(String key, Object value) {
+    // TODO replace all assertions with ArgumentError ?
+    assert(key != null && value != null);
+    assert(value is num || value is bool || value is String);
+
     _contexts[key] = value;
   }