Normalizing runtime ES5 compatibility

[tomrav]

We're currently manually maintaining our runtime code as ES5 compatible.

This can break some of our users when we slip up and introduce code that is not ES5 compatible.

We need to decide if going forward we'd like to:

• simply transpile the entire runtime package to ES5 (and maybe add some checks for size/content?)

OR

• add some validation (eslint?) to check that all code in our runtime is in fact ES5 compliant

Other options may exist.

[idoros]

The reason we originally choose to manually write in ES5 for the runtime was to control the size/performance of the output?