Class Example in Page "Interface"

Hey Basarat,

I'm reading the secsion [***Interface***](https://basarat.gitbooks.io/typescript/docs/types/interfaces.html) of the book. And decided to copy & past some snippet into [***typescript Playground***](https://www.typescriptlang.org/play/index.html). I've find that your code may not designed to be *copyAndPastable*. (See [this result](https://www.typescriptlang.org/play/index.html?ssl=7&ssc=1&pln=7&pc=2#code/JYOwLgpgTgZghgYwgAgAoHtRmQbwLABQyxyAHgFzIgCuAtgEbQDcyAnpTQ84QL6GEIANnADOI5AFlWGLMmC0ADoIi0I4cTPC5CJMhzqMoLdlQPNkAegvIAynFXJRaTOF5A))

As a beginner for `class` in Javascript, I belive here I may initialize `x` and `y` in the constructure. So what's your purpose here? Prefer a concise expression as exist on the book or others?

**BTW**:
Awesome book really! I'm sure I will buy you something after I slowly finish my reading in the new future! 🌹
I'm pretty sure I've been your fans now 💘

Cheers

Su Gao

Read the Interface section of the book and reproduce the linked TypeScript Playground result. Compare the class example with the reported copy-and-paste failure, then clarify the example or its explanation so a beginner understands the intended initialization and can run the snippet successfully.

Written by the indexing model from the issue text.