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.