testRoundtripScopeRefAndPropIter causes 15% performance degradation

For some reason the `testRoundtripScopeRefAndPropIter` causes a 15% performance degradation in *subsequent* benchmarks (as measured by `benchFibbonacci10k`).

Here is the offending test, from `serialize_test.js`:
```js
exports.testRoundtripScopeRefAndPropIter = function(t) {
runTest(t, 'testRoundtripScopeRefAndPropIter', `
var r = 0, o = {a: 1, b: 2};
`,`
for (var k in o) {
r += o[k];
}
`, 'r;', 3, 1, true);
};
```

N.B.: Changing seventh parameter to `runTest`—the number of steps between roundtrips—from `1` to `30` will avoid the performance hit, but setting it to `20` does not—even though in both cases just one roundtrip is done during the test (instead of 33 as would be the case roundtripping every step)!