[BUGFIX] checking whether value key is in descriptor instead of checking if value is truthy since !!0 === false

class Foo {
  static bar = 0;
}

Foo.bar++;
// Cannot assign to read only property 'bar' of function

src/babel/transformation/templates/helper-create-class.js

@@ -4,7 +4,7 @@
       var descriptor = props[i];
       descriptor.enumerable = descriptor.enumerable || false;
       descriptor.configurable = true;
-      if (descriptor.value) descriptor.writable = true;
+      if ("value" in descriptor) descriptor.writable = true;
       Object.defineProperty(target, descriptor.key, descriptor);
     }
   }