I suggest replace current standard library polyfill (es6-shim & es6-symbol) to core.js. Why?

• Current polyfill hasn't WeakMap & WeakSet, but we need WeakMap for abstract references. core.js add simple weak-collections polyfill with store values on keys, it doesn't leak and suitable on most cases.
• core.js add simple module with symbols & methods for abstract references.
• es6-shim replace native Map & Set in all modern browsers. That, given the performance polyfill, not serious. core.js just fix them.
• es6-shim Map & Set store data on object chain - lookup time is O(n). core.js Map & Set store data on hash - lookup time is O(1) (but they can't work with frozen objects). Try simple test on native Map, es6-shim and core-js:

var array = [];
for(var i = 0; i < 100000; i++)array.push([{}, {}]);
array = array.concat(array);
console.time('create Map');
var map = new Map(array);
console.timeEnd('create Map');
console.log('Map size: ' + map.size);
// FF34:
// native:                   177.46мс
// es6-shim:                 462863.9мс
// core.js, fixed native:    155.05мс
// core.js, forced replaced: 2287.16мс

• es6-shim Map & Set doesn't work on IE8-. core.js has fallback for them.
• es6-symbol generate string key like "@@1", possible conflicts. core.js Symbol generate more complex string key.
• es6-symbol symbols are enumerable in for-in loop & Object.keys. core.js define setter in Object.prototype - symbols are not enumerable. In the next version I will add an option to disable it.
• Size: current polyfill browser-polyfill.js size is ~54kb. Custom core.js build with all ES6, ES7 modules, setImmediate (dependence of Promise module) size is:

grunt build:es6,es6_collections,es6_iterators,es6_symbol,es6_promise,es7,es7_refs,immediate uglify
File ./custom.min.js created: 49.43 kB → 15.17 kB → 3.97 kB (gzip)

add ~4.5kb regenerator runtime -> ~19-20kb standard library with much greater capacity than the current. Also you can add another modules or use shim build.

You can try core.js with 6to5 in this simple playground.