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

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index 2f68eff24197a6354d70905baf7422f8635dc598..eefeea03d2a591d6016b49c930fe0c39ccbb7e86 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -32,6 +32,7 @@ utils.Import(function(from) {
 // -------------------------------------------------------------------
 
 function SymbolConstructor(x) {
+  "use strict";

[Michael Starzinger, 2015/09/15 13:09:58]

The entire module is already strict, let's drop it.

   if (%_IsConstructCall()) throw MakeTypeError(kNotConstructor, "Symbol");
   // NOTE: Passing in a Symbol value will throw on ToString().
   return %CreateSymbol(IS_UNDEFINED(x) ? x : ToString(x));