Non-native Symbol when targeting ES6

[kitsonk]

TypeScript complains when using well known symbols that TypeScript cannot determine are from the global Symbol interface. (see microsoft/TypeScript#8169, microsoft/TypeScript#8099, microsoft/TypeScript#5579, microsoft/TypeScript#2012)

We need to provide a viable workaround for our Symbol shim due to this.

[kitsonk]

We have a typing that is loaded in ES5 environments that puts the Symbol in the global namespace. It could be improved when targeting ES6, but it works for now (and Symbol has moved to dojo-shim anyways) so closing...