Fix exception and simplify inheritance

[jcollins-g]

Fixes #2213. The only peculiar bit is how this worked at all, before.

[jcollins-g]

The original exception at a current head SDK. Both this and #2213 should be fixed with this PR.

/Users/jcollins/dart/all_sdks/current/bin/dart --enable-asserts --enable-vm-service:64901 /Users/jcollins/dart/dartdoc/bin/dartdoc.dart
Observatory listening on http://127.0.0.1:64901/UngnCU74uAg=/

Documenting github_actions_toolkit...
Initialized dartdoc with 46 libraries in 16.8 seconds
Generating docs for library github_actions_toolkit from package:github_actions_toolkit/github_actions_toolkit.dart...

dartdoc failed: type 'FunctionTypeElementType' is not a subtype of type 'DefinedElementType'
package:dartdoc/src/model/method.dart 99:39                         Method.modelType
package:dartdoc/src/model/method.dart 96:34                         Method.linkedReturnType
dart:mirrors                                                        _InstanceMirror.getField
package:mustache/src/renderer.dart 245:29                           Renderer._getNamedProperty
package:mustache/src/renderer.dart 212:16                           Renderer.resolveValue
package:mustache/src/renderer.dart 90:17                            Renderer.visitVariable
package:mustache/src/node.dart 45:43                                VariableNode.accept
package:mustache/src/renderer.dart 64:53                            Renderer.render.<fn>
dart:_internal                                                      ListIterable.forEach
package:mustache/src/renderer.dart 64:36                            Renderer.render
package:mustache/src/renderer.dart 195:16                           Renderer.visitPartial
package:mustache/src/node.dart 81:43                                PartialNode.accept
package:mustache/src/node.dart 66:37                                SectionNode.visitChildren.<fn>
dart:core                                                           List.forEach
package:mustache/src/node.dart 66:14                                SectionNode.visitChildren
package:mustache/src/renderer.dart 184:10                           Renderer._renderWithValue
package:mustache/src/renderer.dart 126:28                           Renderer._renderSection.<fn>
dart:core                                                           List.forEach
package:mustache/src/renderer.dart 126:13                           Renderer._renderSection
package:mustache/src/renderer.dart 115:7                            Renderer.visitSection
package:mustache/src/node.dart 63:43                                SectionNode.accept
package:mustache/src/node.dart 66:37                                SectionNode.visitChildren.<fn>
dart:core                                                           List.forEach
package:mustache/src/node.dart 66:14                                SectionNode.visitChildren
package:mustache/src/renderer.dart 184:10                           Renderer._renderWithValue
package:mustache/src/renderer.dart 130:7                            Renderer._renderSection
package:mustache/src/renderer.dart 115:7                            Renderer.visitSection
package:mustache/src/node.dart 63:43                                SectionNode.accept
package:mustache/src/node.dart 66:37                                SectionNode.visitChildren.<fn>
dart:core                                                           List.forEach
package:mustache/src/node.dart 66:14                                SectionNode.visitChildren
package:mustache/src/renderer.dart 184:10                           Renderer._renderWithValue
package:mustache/src/renderer.dart 144:7                            Renderer._renderSection
package:mustache/src/renderer.dart 115:7                            Renderer.visitSection
package:mustache/src/node.dart 63:43                                SectionNode.accept
package:mustache/src/renderer.dart 58:30                            Renderer.render.<fn>
dart:core                                                           List.forEach
package:mustache/src/renderer.dart 58:13                            Renderer.render
package:mustache/src/template.dart 40:14                            Template.render
package:mustache/src/template.dart 33:5                             Template.renderString
package:dartdoc/src/generator/dartdoc_generator_backend.dart 59:28  DartdocGeneratorBackend.render
package:dartdoc/src/generator/dartdoc_generator_backend.dart 112:5  DartdocGeneratorBackend.generateClass
package:dartdoc/src/generator/generator_frontend.dart 61:29         GeneratorFrontEnd._generateDocs
package:dartdoc/src/generator/generator_frontend.dart 23:5          GeneratorFrontEnd.generate
package:dartdoc/dartdoc.dart 166:23                                 Dartdoc.generateDocsBase
===== asynchronous gap ===========================
package:dartdoc/dartdoc.dart 193:32                                 Dartdoc.generateDocs
dart:async                                                          runZoned
../dartdoc/bin/dartdoc.dart 94:13                                   main.<fn>
package:stack_trace                                                 Chain.capture
../dartdoc/bin/dartdoc.dart 93:17                                   main


Process finished with exit code 255

[coveralls]

Coverage decreased (-0.03%) to 93.01% when pulling 7cab23c on fix-exception-modeltype into 3c44922 on master.