Cleanup around SourceFile class  - Simplify custom serialization logic and charset handling  - Removes duplicated default for charset  - Fixes incorrect charset passed in getCodeReader
Cleanup around SourceFile class
 - Simplify custom serialization logic and charset handling
 - Removes duplicated default for charset
 - Fixes incorrect charset passed in getCodeReader

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=227570846

nawawi/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,vobruba-martin/closure-compiler,vobruba-martin/closure-compiler,google/closure-compiler,monetate/closure-compiler,google/closure-compiler,Yannic/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,google/closure-compiler,vobruba-martin/closure-compiler,Yannic/closure-compiler,monetate/closure-compiler,google/closure-compiler,Yannic/closure-compiler,vobruba-martin/closure-compiler,Yannic/closure-compiler,monetate/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler