Merge pull request #49 from falsandtru/es6

Fix a leakage of strict mode

Author: twada

test/tobe_instrumented/es6_test.js

@@ -9,6 +9,7 @@
         factory(root.assert, root.expect);
     }
 }(this, (assert, expect) => {
+    'use strict';
 
     function expectPowerAssertMessage (body, expectedLines) {
         try {