better use strict handling inside of modules

lib/6to5/transformation/modules/system.js

@@ -1,6 +1,7 @@
 module.exports = SystemFormatter;
 
 var AMDFormatter = require("./amd");
+var useStrict    = require("../transformers/use-strict");
 var traverse     = require("../../traverse");
 var util         = require("../../util");
 var t            = require("../../types");
@@ -170,5 +171,9 @@ SystemFormatter.prototype.transform = function (ast) {
 
   handlerBody.push(returnStatement);
 
+  if (useStrict._has(block)) {
+    handlerBody.unshift(block.body.shift());
+  }
+
   program.body = [runner];
 };

lib/6to5/transformation/transform.js

@@ -85,9 +85,9 @@ _.each({
   _declarations:             require("./transformers/_declarations"),
 
   // wrap up
+  useStrict:                 require("./transformers/use-strict"),
   _aliasFunctions:           require("./transformers/_alias-functions"),
   _moduleFormatter:          require("./transformers/_module-formatter"),
-  useStrict:                 require("./transformers/use-strict"),
 
   typeofSymbol:              require("./transformers/optional-typeof-symbol"),
   coreAliasing:              require("./transformers/optional-core-aliasing"),