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
diff --git a/lib/codemirror/BUCK b/lib/codemirror/BUCK
index 8fc6942..dd10d1e 100644
--- a/lib/codemirror/BUCK
+++ b/lib/codemirror/BUCK
@@ -20,6 +20,7 @@
 
 CLOSURE_COMPILER_ARGS = [
   '--compilation_level SIMPLE_OPTIMIZATIONS',
+  '--language_out ECMASCRIPT5_STRICT',
   '--warning_level QUIET'
 ]