experimental support for decorators visual studio 2017
Same issue here and it was driving me mad.
Add in .csproj file of your project
<ItemGroup>
<Content Include="ClientApp\tsconfig.json">
<CopyToOutputDirectory>Never</CopyToOutputDirectory>
</Content>
</ItemGroup>
https://github.com/Microsoft/TypeScript/issues/25823
add
<PropertyGroup>
<TypeScriptExperimentalDecorators>true</TypeScriptExperimentalDecorators></PropertyGroup>
in .csproj file of your project as it will be given precedence over tsconfig.json file and the restart visual studio and the error is no longer there.