Support ARM-native installs of Dart on Chocolatey

[timsneath]

ARM is becoming a thing on Windows, but it doesn't look like our Windows installation instructions support it through their default path.

I believe we are blocked on chocolatey/choco#1803 to fix this.

[timsneath]

@pbo-linaro, are your team working with Chocolatey at all?

[pbo-linaro]

We didn't look into Chocolatey (but we use it with x64 emulation). Chocolatey devs will probably add this in the future.

[sortie]

@rmacnak-google Now that you're working on Windows ARM64` and it reached beta we might want to think about this. Although perhaps we should wait until it reaches stable :)