@mhegazy I chose to be explicit about the check as I wanted to avoid walking up the spine to check for an ambient context every time we call this function. extends currently is the only case where this matters.

I'm also seeing this on destructuring in angular2:
node_modules/@angular/core/src/metadata/di.d.ts(178,48): error TS2354: This syntax requires an imported helper but module 'tslib' cannot be found.

The d.ts it's complaining about is:

    (selector: Type<any> | Function | string, {descendants, read}?: {
        descendants?: boolean;
        read?: any;
    }): any;
    new (selector: Type<any> | Function | string, {descendants, read}?: {
        descendants?: boolean;
        read?: any;
    }): Query;

I don't get why it says tslib cannot be found though. A --traceResolution shows that tslib is succesfully resolved from node_modules folder many times.

@dtabuenc, have you tried with typescript@next?

Yeah I tried with with typescript@next installed minutes before I posted.
Version I used was Version 2.2.0-dev.20161213