Pass "--language_out ECMASCRIPT5_STRICT" to Closure Compiler

This makes Closure Compiler add a "use strict;" directive to the
beginning of the output, which might improve the performance
on JavaScript engines.

Change-Id: I239d08bdbc47e98a373cf1b1d3b0417e36875dea

lib/codemirror/BUCK

@@ -20,6 +20,7 @@ CLOSURE_VERSION = 'v20160208'
 
 CLOSURE_COMPILER_ARGS = [
   '--compilation_level SIMPLE_OPTIMIZATIONS',
+  '--language_out ECMASCRIPT5_STRICT',
   '--warning_level QUIET'
 ]