'not_wasm' prefix doesn't work

[davidjohnoliver]

I'm submitting a...

Bug report (I searched for similar issues and did not find one)

Current behavior

<not_wasm:Style TargetType="Frame" BasedOn="{StaticResource NativeDefaultFrame}" />

Implicit Style is included in WASM head.

Expected behavior

Code is excluded from WASM head.

Minimal reproduction of the problem with instructions

Environment

Nuget Package: 

Package Version(s): 

Affected platform(s):
- [ ] iOS
- [ ] Android
- [x] WebAssembly
- [ ] Windows
- [ ] Build tasks

Visual Studio
- [ ] 2017 (version: )
- [ ] 2017 Preview (version: )
- [ ] for Mac (version: )

Relevant plugins
- [ ] Resharper (version: )