Fix void as a type parameter by rewriting most of element_type.dart (#1629)

* Beginnings of refactor, partially works

* unit tests pass, integration tests fail

* refactoring type arguments part 1

* getting closer.

* closer!

* Works!

* pubspec.lock

* new void tests pt 1

* void tests pt 2

* cleanup part 1

* cleanup part 2

* ModelFunctionAnonymous shift pt1

* cleanups

* dartfmt and pub get

* fix windows only analysis errors

* stack depth reduction

* Fix regression in typedef parameters

Author: jcollins-g

lib/src/element_type.dart

@@ -10,6 +10,7 @@ import 'dart:math';
 
 import 'package:analyzer/dart/ast/ast.dart' hide TypeParameter;
 import 'package:analyzer/dart/element/element.dart';
+import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model_utils.dart';
 import 'package:html/parser.dart' show parse;
 import 'package:markdown/markdown.dart' as md;
@@ -422,7 +423,7 @@ ModelElement _findRefElementInLibrary(String codeRef, Warnable element,
   // Maybe this ModelElement has type parameters, and this is one of them.
   if (results.isEmpty) {
     if (element is TypeParameters) {
-      results.addAll((element as TypeParameters).typeParameters.where((p) =>
+      results.addAll(element.typeParameters.where((p) =>
           p.name == codeRefChomped || codeRefChomped.startsWith("${p.name}.")));
     }
   }
@@ -644,8 +645,9 @@ void _getResultsForClass(Class tryClass, String codeRefChomped,
   // Otherwise, search the class.
   if ((tryClass.modelType.typeArguments.map((e) => e.name))
       .contains(codeRefChomped)) {
-    results.add(tryClass.modelType.typeArguments
-        .firstWhere((e) => e.name == codeRefChomped)
+    results.add((tryClass.modelType.typeArguments.firstWhere(
+                (e) => e.name == codeRefChomped && e is DefinedElementType)
+            as DefinedElementType)
         .element);
   } else {
     // People like to use 'this' in docrefs too.
@@ -655,14 +657,12 @@ void _getResultsForClass(Class tryClass, String codeRefChomped,
       // TODO(jcollins-g): get rid of reimplementation of identifier resolution
       //                   or integrate into ModelElement in a simpler way.
       List<Class> superChain = [tryClass];
-      superChain
-          .addAll(tryClass.interfaces.map((t) => t.returnElement as Class));
+      superChain.addAll(tryClass.interfaces.map((t) => t.element as Class));
       // This seems duplicitous with our caller, but the preferredClass
       // hint matters with findCanonicalModelElementFor.
       // TODO(jcollins-g): This makes our caller ~O(n^2) vs length of superChain.
       //                   Fortunately superChains are short, but optimize this if it matters.
-      superChain
-          .addAll(tryClass.superChain.map((t) => t.returnElement as Class));
+      superChain.addAll(tryClass.superChain.map((t) => t.element as Class));
       List<String> codeRefParts = codeRefChomped.split('.');
       for (final c in superChain) {
         // TODO(jcollins-g): add a hash-map-enabled lookup function to Class?