That error will appear until you upgrade to TypeScript 2.0. Even though the error appears, I did not identify any adverse behaviors other than compile/lint warnings (red squiggles) with classes that have decorators. That particular issue is likely an incompatibility with a typings definition present in 1.8 that changed in 2.0. I upgraded to TypeScript 2.0 beta and all my projects are fine. TypeScript 2.0 beta has been around for a while appears to be stable.
npm yells anytime it finds a missing dependency missing or version mismatch. You can always check first level dependency packages/versions installed in a project or globally with the following respectively:
npm ls --depth=0
npm ls -g --depth=0