[interop] Support Classes and Interfaces

[nikeokoronkwo]

Fixes #402
Fixes #409
Fixes #411

This PR adds support for classes and interfaces in TypeScript for the Dart JS Interop Interface Generator.

Interfaces and Classes in TS are mapped to Dart extension types, following Dart's new JS Static Interop for JS. Constructors are generated by default for classes (defaulting to a zero-parameter constructor if not specified). Unless denoted by hierarchy, all extension types by default have JSObject as their rep type and implement JSObject

1. Constructors are mapped to Dart constructors on an extension type
2. Methods are mapped to Dart methods
3. Properties are mapped to Dart properties
4. get and set accessors are mapped to getters and setters in Dart
5. callable interfaces are mapped to extension types with a call parameter, so they can be called in Dart.
6. indexable interfaces and classes are mapped to override the operators of [] and []= (the second is only if the indexer member is not readonly), so they can be indexed in Dart.
7. readonly members are mapped to only getters rather than normal fields
8. Only public members are generated, as those are the only members that can be accessed by Dart's JS Interop layer.
9. Alternate constructors are mapped to factory constructors with different names in the extension type
10. Overriden methods are marked with @redeclare
11. Nullable Methods are represented as getters that return JSFunction? (once dart-lang/sdk#54557 lands, we will have it with generics)
12. Extended classes are represented with the extendee being implemented as well as being it's representation type
13. Implemented classes and extended interfaces are represented with the extendees/implementees being implemented on the extension type.

Therefore, the following

interface Shape<TMetadata = any> {
    readonly id: string;
    name: string;
    area(): number;
    area(unit: 'cm2' | 'in2'): string;
    onUpdate?(prev: Shape<TMetadata>): void;
}
declare class ShapeImpl<TMeta = any> implements Shape<TMeta> {
    readonly id: string;
    name: string;
    /* other decls in Shape */
    metadata?: TMeta;
    constructor(name: string, type?: 'circle' | 'rectangle' | 'polygon');
    constructor(config: Shape<TMeta>);
    get location(): string;
    set location(value: string);
    static getById(id: string): ShapeImpl;
    toString(): string;
}

Maps to this

import 'dart:js_interop' as _i1;
import 'package:meta/meta.dart' as _i2; // needed for @redeclare

extension type Shape<TMetadata extends _i1.JSAny>(_i1.JSObject _) implements _i1.JSObject {
  external String get id;
  external String name;
  external double area();
  @_i1.JS('area')
  external String area$1(AnonymousUnion unit); // AnonymousUnion defined afterwards
  external JSFunction? get onUpdate;
}

extension type ShapeImpl <TMeta extends _i1.JSAny>(_i1.JSObject _) implements Shape {
  external ShapeImpl(String name, [AnonymousUnion$1 type]);
  external factory ShapeImpl.unnamed$1(Shape<TMeta> config);
  @_i2.redeclare
  external String get id;
  external String name;

  external TMeta? metadata;
  external String get location;
  external set location(String value);
  external static ShapeImpl getById(String id)
  /* other decls in Shape */
}

For more examples, check out the added tests classes_input.d.ts and interfaces_input.d.ts as well as their expected results for more

Other features, such as an adjusted AST, filtering mechanism, and more have been implemented.

This PR also adds support for a typeMap to allow reusage of certain types with associated declarations such as unions

[srujzs]

Awesome work!!! This is super cool to see - great PR description and tests!

[kevmoo]

good to merge @srujzs ?

[srujzs]

Looks like you were able to unify the various class and interface code, awesome!