support class assigned to var

[timofei-iatsenko]

Support case when class expression assigned to a variable.

let Foo = class Foo {
      constructor($element) {
        'ngInject';
      }
    };

Closes https://github.com/schmod/babel-plugin-angularjs-annotate/issues/44

That pattern typically happens in generated code of classes + decorators, when plugin piped after typescript compiler.

[timofei-iatsenko]

@schmod please review and merge