Bug?

looptest.html

    var b;
    var o = {b : 1};
    var count1 = 0, count2 = 0, count3 = 0;
    console.time('global');
    while (b = 1) {
        if (count1 > 1000*1000) break;
        count1 += b;
    }
    console.timeEnd('global');
    console.time('global object');
    while (true) {
        if (count2 > 1000*1000) break;
        count2 += o.b;
    }
    console.timeEnd('global object');
    console.time('local');
    while (true) {
        var c = 1;
        if (count3 > 1000*1000) break;
        count3 += c;
    }
    console.timeEnd('local');
    console.log(count1, count2, count3);

IE 11:
global: 3.6150ms
global object: 3.4786ms
local: 3.4637ms

Chrome Canary:
global: 14.435ms
global object: 3.006ms
local: 218.870ms

FF:
global: 3.22ms
global object: 2.87ms
local: 2.78ms

FF (nighly):
global: 3.33ms
global object: 2.82ms
local: 2.79ms