noUnusedLocals flags decorated classes

[roblourens]

TypeScript Version: 2.1.5 and latest 2.2.0

Code

@f
class A {
    private b;

    c() {

    }
}

export function f(ctor) {
    new ctor().c();
}

with tsconfig.json:

{
    "compilerOptions": {
        "noUnusedLocals": true,
        "target": "es6",
        "module": "commonjs",
        "experimentalDecorators": true
    }
}

Expected behavior:
A.b should be flagged as unused, but A should not be flagged as unused, because the decorator could be doing something useful with the class. This is a common pattern in our codebase.

Actual behavior:
A is flagged as unused

[mhegazy]

Duplicate of #13120