Export the 'then' property of a Thenable without using goog.exportProperty.
Export the 'then' property of a Thenable without using goog.exportProperty.

This allows us to remove an ugly hack in //chrome/dongle/ because the "generate externs for exports" pass no longer creates a "var ctor;" in the externs file that it outputs.

RELNOTES: none

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134568820

codyebberson/closure-library,ChadKillingsworth/closure-library,joeltine/closure-library,lucidsoftware/closure-library,joeltine/closure-library,teppeis/closure-library,google/closure-library,codyebberson/closure-library,lucidsoftware/closure-library,google/closure-library,Medium/closure-library,platinumpixs/google-closure-library,codyebberson/closure-library,joeltine/closure-library,ikabirov/closure-library,joeltine/closure-library,platinumpixs/google-closure-library,redforks/closure-library,Medium/closure-library,ikabirov/closure-library,lucidsoftware/closure-library,redforks/closure-library,Medium/closure-library,ikabirov/closure-library,redforks/closure-library,google/closure-library,Medium/closure-library,teppeis/closure-library,ChadKillingsworth/closure-library,teppeis/closure-library,platinumpixs/google-closure-library,ChadKillingsworth/closure-library,codyebberson/closure-library,redforks/closure-library,google/closure-library,ikabirov/closure-library,ChadKillingsworth/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,google/closure-library,lucidsoftware/closure-library