Externs aren't generated for type aliases of records

[samadlotia]

The Google Maps typings file declares two types as:

export type LatLngLiteral = { lat: number; lng: number }
export type LatLngBoundsLiteral = { east: number; north: number; south: number; west: number }

tsickle then produces this in the externs file:

/** @typedef {?} */    
google.maps.LatLngLiteral;

/** @typedef {?} */      
google.maps.LatLngBoundsLiteral;

That causes the fields in these two types to be obfuscated by the Closure compiler.

However, if I change the typings file like so:

export interface LatLngLiteral { lat: number; lng: number }
export interface LatLngBoundsLiteral { east: number; north: number; south: number; west: number }

tsickle then produces this in the externs file:

/** @record @struct */    
google.maps.LatLngLiteral = function() {};    

 /** @type {?} */    
google.maps.LatLngLiteral.prototype.lat;    

 /** @type {?} */    
google.maps.LatLngLiteral.prototype.lng;         

/** @record @struct */    
google.maps.LatLngBoundsLiteral = function() {};    

 /** @type {?} */    
google.maps.LatLngBoundsLiteral.prototype.east;    

 /** @type {?} */    
google.maps.LatLngBoundsLiteral.prototype.north;    

 /** @type {?} */    
google.maps.LatLngBoundsLiteral.prototype.south;    

 /** @type {?} */    
google.maps.LatLngBoundsLiteral.prototype.west;

This prevents the field names from becoming obfuscated.

[tbosch]

Note: This might be fixed by the upcoming transformer version of tsickle (see #516), where we generate explicit reexports for all types (including interfaces).