When composing source maps, use the original mapping even if the source file cannot be located
When composing source maps, use the original mapping even if the source file cannot be located

Closes https://github.com/google/closure-compiler/pull/2921

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=197595899

mprobst/closure-compiler,ChadKillingsworth/closure-compiler,Yannic/closure-compiler,tdelmas/closure-compiler,shantanusharma/closure-compiler,google/closure-compiler,tiobe/closure-compiler,tiobe/closure-compiler,vobruba-martin/closure-compiler,nawawi/closure-compiler,shantanusharma/closure-compiler,monetate/closure-compiler,google/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,tiobe/closure-compiler,shantanusharma/closure-compiler,monetate/closure-compiler,google/closure-compiler,mprobst/closure-compiler,ChadKillingsworth/closure-compiler,Yannic/closure-compiler,tiobe/closure-compiler,vobruba-martin/closure-compiler,google/closure-compiler,nawawi/closure-compiler,tdelmas/closure-compiler,vobruba-martin/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,mprobst/closure-compiler,vobruba-martin/closure-compiler,shantanusharma/closure-compiler,Yannic/closure-compiler,monetate/closure-compiler,tdelmas/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,tdelmas/closure-compiler,mprobst/closure-compiler