Missing polyfills

• ES BigInt can't be polyfilled since it requires changes in the behavior of operators, you can find more info here. You could try to use JSBI.
• ES Proxy can't be polyfilled, you can try to use proxy-polyfill which provides a very small subset of features.
• ES String#normalize is not a very useful feature, but this polyfill will be very large. If you need it, you can use unorm.
• ECMA-402 Intl is missed because of the size. You can use those polyfills.
• window.fetch is not a cross-platform feature, in some environments, it makes no sense. For this reason, I don't think it should be in core-js. Looking at a large number of requests it might be added in the future. Now you can use, for example, this polyfill.