Interop Gen: Support Constructor Types

[nikeokoronkwo]

This issue is to add support for TypeScript Constructor Types in the Dart TS Interop Interface Generator.

// assuming there is a class called `MyClass` defined
export declare const myConstructor: new () => MyClass;

This will be helpful when working with namespaces (see #399), as translating classes defined in namespaces would mean interfacing class calls as constructor type calls.