Fixes dart-lang#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 (dart-lang#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
```