Fix exception and simplify inheritance (#2214)

Author: jcollins-g

lib/src/model/method.dart

@@ -5,7 +5,6 @@
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/src/dart/element/member.dart' show Member;
-import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/model.dart';
 
 class Method extends ModelElement
@@ -95,9 +94,6 @@ class Method extends ModelElement
 
   String get linkedReturnType => modelType.createLinkedReturnTypeName();
 
-  @override
-  DefinedElementType get modelType => super.modelType;
-
   @override
   Method get overriddenElement {
     if (_enclosingContainer is Extension) {

lib/src/model/type_parameter.dart

@@ -69,7 +69,7 @@ class TypeParameter extends ModelElement {
   TypeParameterElement get _typeParameter => element as TypeParameterElement;
 }
 
-abstract class TypeParameters implements ModelElement {
+mixin TypeParameters implements ModelElement {
   String get nameWithGenerics => '$name$genericParameters';
 
   String get nameWithLinkedGenerics => '$name$linkedGenericParameters';
@@ -82,9 +82,6 @@ abstract class TypeParameters implements ModelElement {
   String get linkedGenericParameters =>
       _typeParametersRenderer.renderLinkedGenericParameters(this);
 
-  @override
-  DefinedElementType get modelType;
-
   List<TypeParameter> get typeParameters;
 
   TypeParametersRenderer get _typeParametersRenderer =>