[interop] Add Support for Anonymous Declarations

[nikeokoronkwo]

Fixes #385
Fixes #422
Fixes #410
Fixes #433

This PR adds support for generating declarations for anonymous declarations, such as anonymous objects, unions, closures and constructor types.

This PR also adds support for nullable types (types unioned with undefined and/or null).

A hashing function is used for consistently hashing string objects, which is used for hashing identifiers for anonymous unions, objects and more, as well as comparing such names to allow reusing of such types.

• Anonymous Unions are turned into a Dart (anonymous) extension type which has a rep type/implements the shared type between the union parts. It has getters to get the value as a certain probability of the union. For instance, a union string | number | boolean may look like the following:

  extension type _AnonymousUnion_204398._(JSAny _) implements JSAny {
    String get asString => (_ as JSString).toDart;
    double get asDouble => (_ as JSNumber).toDartDouble;
    bool get asBool => (_ as JSBoolean).toDart;
  }

• Anonymous Tuples are turned into an instance of a JSTupleX declaration generated depending on the number of members it may have (i.e if a 2 member tuple exists, it will be an instance of JSTuple2<A, B>). The tuple extension type looks like the following:

  // normal
  extension type JSTupleX<A, B, ..., B1>._(JSArray<JSAny> _) implements JSArray<JSAny> {
    A get $1 => _[0] as A;
    B get $2 => _[1] as B;
    /* ... */
    B1 get $x => _[x] as B1;
    
    set $1(A newValue) => _[0] = newValue;
    set $2(B newValue) => _[1] = newValue;
    /* ... */
    set $x(B1 newValue) => _[x] = newValue;
  }
  
  // readonly
  extension type JSReadonlyTupleX<A, B, ..., B1>._(JSArray<JSAny> _) implements JSArray<JSAny> {
    A get $1 => _[0] as A;
    B get $2 => _[1] as B;
    /* ... */
    B1 get $x => _[x] as B1;
  }

• Anonymous Closures are turned into extension types implementing call. While Anonymous function types ((str: string) => void) have their call implementations as external, anonymous constructor types (new (str: string) => T to construct T) are implemented as function calls to create the given object. In the future after Make JSExportedDartFunction generic and add a toJSGeneric function sdk#54557, we should be able to replace this with a generic JSFunction.

  extension type _AnonymousFunction_123456._(JSFunction _) implements JSFunction {
    external void call(String str);
  }
  
  // Assuming T is defined elsewhere and is **not** a generic
  extension type _AnonymousFunction_123456._(JSFunction _) implements JSFunction {
    T call(String str) => T(str);
  }

• Anonymous Objects are implemented as extension type objects with their default constructor as an object literal constructor. They also inherit any associated generic types alongside.

[srujzs]

Thanks, Nike! Just a couple more comments.

[srujzs]

Sorry, GitHub UI is atrocious and didn't show me all the unresolved comments in the commits tab so I have a few more. :)

[srujzs]

Looks good! Just some small nits around prints.