Include extension members on the extended type's page

Fixes dart-lang#2021

In this feature, we list extension members of an extended type on that extended
type's page. The members are listed inline with other instance members and are
cross-linked over to the extension's member pages.

* Introduce `availableInstanceMethods` as a list combining both instance
  methods declared in a container, and instance methods declared in an
  applicable extension. The same for `availableInstanceOperators` and
  `availableInstanceFields`.
* Introduce some constructors like `Field.providedByExtension` for members that
  are provided by an extension.
* Add lots of documentation clarifying _where_ various members come from.
* We introduce a new getter on ElementType, `nameWithGenericsPlain`, which is
  not HTML-formatted, for use in HTML "title text."

Author: srawlins

lib/resources/styles.css

@@ -595,6 +595,22 @@ h1 .category {
   vertical-align: middle;
 }
 
+/* Do not display "provided by X extension" text on extension pages. */
+.main-content.extension-page .from-extension {
+  display: none;
+}
+
+sup.muted {
+  color: var(--main-sidebar-color);
+  font-size: 0.6em;
+}
+
+.from-extension > span {
+  background-color: var(--alert-warning);
+  font-style: italic;
+  padding: 2px;
+}
+
 /* The badge under a declaration for things like "const", "read-only", etc. and for the badges inline like sealed or interface */
 /* See https://github.com/dart-lang/dartdoc/blob/main/lib/src/model/feature.dart */
 .feature {

lib/src/element_type.dart

@@ -48,9 +48,13 @@ abstract class ElementType with CommentReferable, Nameable {
 
   String get linkedName;
 
-  /// Name with generics and nullability indication.
+  /// Name with generics and nullability indication, in HTML tags.
   String get nameWithGenerics;
 
+  /// Name with generics and nullability indication, in plain text, with
+  /// unescaped angle brackets.
+  String get nameWithGenericsPlain;
+
   @override
   String get displayName => throw UnimplementedError();
 
@@ -103,11 +107,14 @@ class UndefinedElementType extends ElementType {
     return type.documentableElement!.name!;
   }
 
+  @override
+  String get linkedName => name;
+
   @override
   String get nameWithGenerics => '$name$nullabilitySuffix';
 
   @override
-  String get linkedName => name;
+  String get nameWithGenericsPlain => '$name$nullabilitySuffix';
 
   @override
   Iterable<ElementType> get typeArguments => const [];
@@ -241,6 +248,9 @@ class TypeParameterElementType extends DefinedElementType {
 
   @override
   String get nameWithGenerics => '$name$nullabilitySuffix';
+
+  @override
+  String get nameWithGenericsPlain => '$name$nullabilitySuffix';
 }
 
 /// An [ElementType] associated with an [Element].
@@ -341,6 +351,10 @@ mixin Rendered implements ElementType {
   @override
   late final String nameWithGenerics = _renderer.renderNameWithGenerics(this);
 
+  @override
+  late final String nameWithGenericsPlain =
+      _renderer.renderNameWithGenerics(this, plain: true);
+
   ElementTypeRenderer<ElementType> get _renderer;
 }