Dropping pollyfils for ancient browser versions

[Sam Clegg]

Emscripten supports older browsers via `-sLEGACY_BROWSER_SUPPORT` or via `-sMIN_XXX_VERSION` settings.

However there are limits to how far you can go back.  In the next release we are proposing upping these limits (just a little) in order to remove our polyfills for some very old things like `Promise` and `Object.assign`.

See https://github.com/emscripten-core/emscripten/pull/23118.

The above change would update the oldest possible browsers that can be targeted as follows:

Chrome 32 -> 45

Firefox 34 -> 40

Safari 9.0 -> 10.1

These new miniums were all released over 8 years ago and the assumption is that nobody is actually targeting those with Wasm (even via wasm2js).   Is this assumption correct?  Are there folks still targeting these older versions?   

cheers,

sam