support es6.constants by default. Fixes #2932

Summary: In javascriptcore(ios9), this code will run as expected(output 0 1):

```js
for(let i=0; i<2; i++) {
  const data = i;
  console.log(data);
}
```

But when debug in chrome, the above code will fail without `use strict` prologue (if you add prologue, rn will fail with red screen `Const declarations are not supported in strict mode`): https://code.google.com/p/v8/issues/detail?id=4432.

So it's better to transpile contant by default.

Closes https://github.com/facebook/react-native/pull/2955

Reviewed By: @​svcscm

Differential Revision: D2483398

Pulled By: @vjeux

react-packager/.babelrc

@@ -6,9 +6,8 @@
   "whitelist": [
     "es6.arrowFunctions",
     "es6.blockScoping",
-    // This is the only place where we differ from transformer.js
-    "es6.constants",
     "es6.classes",
+    "es6.constants",
     "es6.destructuring",
     "es6.parameters",
     "es6.properties.computed",

transformer.js

@@ -35,6 +35,7 @@ function transform(src, filename, options) {
       'es6.arrowFunctions',
       'es6.blockScoping',
       'es6.classes',
+      'es6.constants',
       'es6.destructuring',
       'es6.parameters',
       'es6.properties.computed',