Generate docs for enum static methods.

Fixes #3584

The GeneratorFrontEnd code was really long, and I think that made it easy to omit things. I've refactored it to reuse more code. Also, the repeated code had bugs:

* For extensions and extension types, we likely _over_-documented, because we didn't omit things that we're canonical.
* For enums, we omitted static methods and static fields (#3584).

Also in this change:

* Change some `clazz` and `eNum` parameter names to `class_` and `enum_`. More standard, and standard inside the analyzer.
* Migrate methods_test.dart to use test_reflective_loader.
* Add method tests for methods on classes, enums, mixins, extensions, and extension types.
* In the shared testing code, add some code that prints out the in-memory files when a file cannot be read. Here's a snippet of what that looks like:

```none
/temp/method_test/doc/
   ├─ index.html
   ├─ __404error.html
   ├─ search.html
   ├─ lib/
   │  ├─ lib-library.html
   │  ├─ lib-library-sidebar.html
   │  ├─ E-class.html
   │  ├─ E-enum-sidebar.html
   │  └─ E/
   │  │  ├─ values-constant.html
```

lib/src/generator/file_structure.dart

@@ -24,7 +24,8 @@ abstract class FileStructure {
         // This should be the common case.
         FileStructure._fromModelElement(documentable),
       _ => throw UnimplementedError(
-          'Tried to build a FileStructure for an unknown subtype of Documentable:  ${documentable.runtimeType}')
+          'Tried to build a FileStructure for an unknown subtype of '
+          "Documentable: '${documentable.runtimeType}'")
     };
   }
 
@@ -42,6 +43,8 @@ abstract class FileStructure {
     return switch (modelElement) {
       Library() => FileStructureImpl(modelElement.dirName, 'library'),
       Mixin() => FileStructureImpl(modelElement.name, 'mixin'),
+      // Probably just an archaic state, but enums do not have a file suffix.
+      Enum() => FileStructureImpl(modelElement.name, null),
       Class() => FileStructureImpl(modelElement.name, 'class'),
       ExtensionType() => FileStructureImpl(modelElement.name, 'extension-type'),
       Operator() => FileStructureImpl(

lib/src/generator/generator_backend.dart

@@ -125,11 +125,11 @@ abstract class GeneratorBackend {
     runtimeStats.incrementAccumulator('writtenCategoryFileCount');
   }
 
-  /// Emits documentation content for the [clazz].
-  void generateClass(PackageGraph packageGraph, Library library, Class clazz) {
-    var data = ClassTemplateData(options, packageGraph, library, clazz);
+  /// Emits documentation content for the [class_].
+  void generateClass(PackageGraph packageGraph, Library library, Class class_) {
+    var data = ClassTemplateData(options, packageGraph, library, class_);
     var content = templates.renderClass(data);
-    write(writer, clazz.filePath, data, content);
+    write(writer, class_.filePath, data, content);
     runtimeStats.incrementAccumulator('writtenClassFileCount');
   }
 
@@ -143,11 +143,11 @@ abstract class GeneratorBackend {
     runtimeStats.incrementAccumulator('writtenConstructorFileCount');
   }
 
-  /// Emits documentation content for the [eNum].
-  void generateEnum(PackageGraph packageGraph, Library library, Enum eNum) {
-    var data = EnumTemplateData(options, packageGraph, library, eNum);
+  /// Emits documentation content for the [enum_].
+  void generateEnum(PackageGraph packageGraph, Library library, Enum enum_) {
+    var data = EnumTemplateData(options, packageGraph, library, enum_);
     var content = templates.renderEnum(data);
-    write(writer, eNum.filePath, data, content);
+    write(writer, enum_.filePath, data, content);
     runtimeStats.incrementAccumulator('writtenEnumFileCount');
   }
 
@@ -189,9 +189,9 @@ abstract class GeneratorBackend {
 
   /// Emits documentation content for the [method].
   void generateMethod(PackageGraph packageGraph, Library library,
-      Container clazz, Method method) {
+      Container container, Method method) {
     var data =
-        MethodTemplateData(options, packageGraph, library, clazz, method);
+        MethodTemplateData(options, packageGraph, library, container, method);
     var content = templates.renderMethod(data);
     write(writer, method.filePath, data, content);
     runtimeStats.incrementAccumulator('writtenMethodFileCount');
@@ -215,9 +215,9 @@ abstract class GeneratorBackend {
 
   /// Emits documentation content for the [field].
   void generateProperty(PackageGraph packageGraph, Library library,
-      Container clazz, Field field) {
+      Container container, Field field) {
     var data =
-        PropertyTemplateData(options, packageGraph, library, clazz, field);
+        PropertyTemplateData(options, packageGraph, library, container, field);
     var content = templates.renderProperty(data);
     write(writer, field.filePath, data, content);
     runtimeStats.incrementAccumulator('writtenPropertyFileCount');