Fix error on extends in declaration file with importHelpers

[rbuckton]

The call to checkExternalEmitHelpers for the __extends helper was incorrectly reporting an error when extends was used in a declaration file with --importHelpers and a missing tslib.d.ts file.

Fixes #12724

[mhegazy]

Actually i think we should push this check into checkExternalEmitHelpers to avoid having to do this for every helper that may be needed in the future, e.g. ambient decrators, or spread/set in type position.

[rbuckton]

@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.

[dtabuenc]

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.

[mhegazy]

@dtabuenc, have you tried with typescript@next?

[dtabuenc]

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