Moves all input IO into a dedicated pass for better time measurement.
Moves all input IO into a dedicated pass for better time measurement.

Previously IO and parsing occurred in the same pass which made it hard to
know if performance fluctuation was caused by CPU contention or IO
contention. After this change it should be possible to know if IO
performance is the culprit.

Tracer mode now looks like:
Log:
pass,runtime,allocMem,codeChanged,astReduction,reduction,gzReduction,astSize,size,gzSize
readInputs,319,42,false,0,0,0,0,0,0
parseInputs,3851,88,false,0,0,0,0,0,0
beforeStandardChecks,1,119,false,0,0,0,0,0,0
checkJsDoc,307,119,false,0,0,0,0,0,0
...

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146417464

mprobst/closure-compiler,shantanusharma/closure-compiler,google/closure-compiler,tdelmas/closure-compiler,mprobst/closure-compiler,Dominator008/closure-compiler,google/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,anomaly/closure-compiler,monetate/closure-compiler,Pimm/closure-compiler,GerHobbelt/closure-compiler,Yannic/closure-compiler,tiobe/closure-compiler,monetate/closure-compiler,anomaly/closure-compiler,shantanusharma/closure-compiler,mprobst/closure-compiler,anomaly/closure-compiler,Dominator008/closure-compiler,vobruba-martin/closure-compiler,mprobst/closure-compiler,tdelmas/closure-compiler,nawawi/closure-compiler,GerHobbelt/closure-compiler,Pimm/closure-compiler,GerHobbelt/closure-compiler,Yannic/closure-compiler,brad4d/closure-compiler,Yannic/closure-compiler,Pimm/closure-compiler,MatrixFrog/closure-compiler,google/closure-compiler,vobruba-martin/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,tiobe/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,GerHobbelt/closure-compiler,google/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,shantanusharma/closure-compiler,ChadKillingsworth/closure-compiler,tiobe/closure-compiler,MatrixFrog/closure-compiler,brad4d/closure-compiler,tdelmas/closure-compiler,nawawi/closure-compiler,anomaly/closure-compiler,Dominator008/closure-compiler,vobruba-martin/closure-compiler,tiobe/closure-compiler,brad4d/closure-compiler,shantanusharma/closure-compiler,tdelmas/closure-compiler,ChadKillingsworth/closure-compiler,MatrixFrog/closure-compiler,MatrixFrog/closure-compiler