Let build.js handle the "use strict" directive and not minify.js.

Former-commit-id: 741eb692b158e22aa688d6dac1b63fc2787cc426

build/minify.js

@@ -25,7 +25,6 @@
   /** Closure Compiler command-line options */
   var closureOptions = [
     '--compilation_level=ADVANCED_OPTIMIZATIONS',
-    '--language_in=ECMASCRIPT5_STRICT',
     '--warning_level=QUIET'
   ];