[runtime] Fix effect of setting .prototype on generator functions.

test/mjsunit/es6/generators-objects.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 69 matching lines @@
     assertThrows(function() { iter.throw.call(non_generator, 1); }, TypeError);
   }
 
   TestNonGenerator(1);
   TestNonGenerator({});
   TestNonGenerator(function(){});
   TestNonGenerator(g);
   TestNonGenerator(g.prototype);
 }
 TestGeneratorObjectMethods();
+
+
+function TestPrototype() {
+  function* g() { }
+
+  let g_prototype = g.prototype;
+  assertEquals([], Reflect.ownKeys(g_prototype));
+
+  let generator_prototype = Object.getPrototypeOf(g_prototype);
+  assertSame(generator_prototype, Object.getPrototypeOf(g).prototype);
+
+  // Unchanged .prototype
+  assertSame(g_prototype, Object.getPrototypeOf(g()));
+
+  // Custom object as .prototype
+  {
+    let proto = {};
+    g.prototype = proto;
+    assertSame(proto, Object.getPrototypeOf(g()));
+  }
+
+  // Custom non-object as .prototype
+  g.prototype = null;
+  assertSame(generator_prototype, Object.getPrototypeOf(g()));
+}
+TestPrototype();