code: public void assignMutableBinding(ExecutionContext context, String name, Object value, boolean configurable, boolean strict) {
          final boolean exists = hasBinding(context, name);
          if (!exists) {
              PropertyDescriptor desc = new PropertyDescriptor();
              desc.setValue(value);
              desc.setConfigurable(configurable);
              this.mutableBindings.put(name, desc);
          } else {
              PropertyDescriptor desc = this.mutableBindings.get(name);
              desc.setValue(value);
              return;
          }
      }
docs: FIXME: describe spec deviance