Update operation .length computation to match spec
Update operation .length computation to match spec

https://bugzilla.mozilla.org/show_bug.cgi?id=777061

bboyle/testharness.js,RatneshThakur/testharness.js,hallvors/testharness.js,zqzhang/testharness.js,ayg/testharness.js,jyasskin/testharness.js,jyasskin/testharness.js,hallvors/testharness.js