Corrected bug whereby multiple override calls would clear __super too early

Before this change, __super was set to undefined. However, consider that we have two
method overrides---foo and bar---and the code for bar is:

  this.foo();
  this.__super();

foo() would set __super to undefined and so bar cannot invoke its super method
unless it stores a reference to __super before invoking foo(). This patch fixes
this issue.