Use native Object.assign when available. Fixes #54.

[mwistrand]

• lang.assign delegates to Object.assign when avaialble.
• Implement has('object-assign') test.

[kitsonk]

An alternative way of doing this, instead of the any cast (that allows the interface to be preserved too) is the following:

interface ObjectAssignConstructor extends ObjectConstructor {
    assign(target: {}; ...sources: {}[]): {};
}

function assign(target: {}; ...sources: {}[]): {} {
    return (typeof (<ObjectAssignConstructor> Object).assign === 'function') ? (<ObjectAssignConstructor> Object).assign.apply(null, arguments) : _mixin({
        deep: false,
        inherited: false,
        sources: sources,
        target: target
    });

[kitsonk]

Oh, also, export const right?

[kitsonk]

@mwistrand the problem you are running into is what I ran into on microsoft/TypeScript#3889. It isn't static methods, it is that you are trying to extend it within the module and you cannot access the global namespace for interfaces from within a module.

[mwistrand]

@kitsonk Thanks for passing along that TS issue. I was partly hoping that built-in interfaces could be opened up for extension (like modules/classes in Ruby), but that's clearly not the case. I'm not convinced, though, that declare var Object: ObjectAssignConstructor is more elegant than casting Object, so I stuck with your initial suggestion.

[kitsonk]

I think they will be, once microsoft/TypeScript#983 is solved. You can extend them if you aren't in a module. The other solution is to add them to the projects .d.ts, but then you are "changing" the global Object for that project, which you might not actually want to do. So the "limitation" in this case actually benefits I think for feature detection.