Add test for JS_CONST

Add test for JS_CONST

BUG=
R=floitsch@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39308

[floitsch, 2014-08-15 17:52:18]

LGTM.

[floitsch, 2014-08-15 17:53:28]

On 2014/08/15 17:52:18, floitsch wrote:
> LGTM.

But it would make sense to use the result in a const object too:

class A {
  final x;
  const A(this.x);
}

main() {
  var a = const A(JS_CONST(...));
  Expect.equals(..., a.x);
}

[sra1, 2014-08-15 18:33:01]

Committed patchset #1 manually as 39308 (presubmit successful).

[sra1, 2014-08-15 18:37:08]

On 2014/08/15 17:53:28, floitsch wrote:
> On 2014/08/15 17:52:18, floitsch wrote:
> > LGTM.
> 
> But it would make sense to use the result in a const object too:
> 
> class A {
>   final x;
>   const A(this.x);
> }
> 
> main() {
>   var a = const A(JS_CONST(...));
>   Expect.equals(..., a.x);
> }

The test just covers current usage.

I'd like to find some other solution for this since these tests are inconsistent
depending whether optimized by type inference to executed dynamically.

var a = JS_CONST(...);
...
a is JS_CONST
a is JavaScriptObject