[es6] Move builtin constructors for primitives to strict mode.

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index a8a9e8bd3af67f0878f4a8b34ce9c8cf49793c6d..e6671eec17d991ce8d4a5ae9a8e800bd76f8ead6 100644
--- a/src/string.js
+++ b/src/string.js
@@ -36,6 +36,8 @@ utils.Import(function(from) {
 //-------------------------------------------------------------------
 
 function StringConstructor(x) {
+  // TODO(bmeurer): Move this to toplevel.
+  "use strict";
   if (%_ArgumentsLength() == 0) x = '';
   if (%_IsConstructCall()) {
     %_SetValueOf(this, TO_STRING_INLINE(x));