anchor.diff:  === :optimize-constants
   
   When set to `true`, constants, such as keywords and symbols, will only
  -be created once and will be written to a separate file called
  -`constants_table.js`. The compiler will emit a reference to the constant
  +be created once and will be written to a separate file
  +(`cljs/core/constants.js`). The compiler will emit a reference to the constant
   as defined in the constants table instead of creating a new object for
   it. This option is mainly intended to be used for a release build since
   it can increase performance due to decreased allocation. Defaults to
  
positive.code: ----
  :language-in  :ecmascript3
  :language-out :ecmascript3
  ----
  
  [[closure-warnings]]
  
negative.code: [[language-in-and-language-out]]
  === :language-in and :language-out
  
  Configure the input and output languages for the closure library. May be:
  
  * `:ecmascript6` identical to `:es6`
  * `:ecmascript6-strict` identical to `:es6-strict`
  * `:ecmascript6-typed` identical to `:es6-typed`
  * `:ecmascript5` identical to `:es5`
  * `:ecmascript5-strict` identical to `:es5-strict`
  * `:ecmascript3` identical to `:es3`
  * `:no-transpile`
  
  Defaults to `:ecmascript3`
  
  