Support augmentation-declared constructors and methods.

The gist of this is to not just ask an Element for it's methods and for its
constructors, but make sure to ask the augmented element, if there is one.

A few fixes are made additionally:

* While working on this, I saw that a crash would mean that the
  AnalysisContextCollectionImpl was never `disposed`. So I tidied that all back
  into PackageBuilder. It can not be the PackageBuilder's caller's
  responsibility to dispose, because that amounts to have a dozen places in
  tests.
* In the code that gathers inheritance info, we also need to make sure to grab
  an element's augmented declaration, if there is one.

lib/src/dartdoc.dart

@@ -225,7 +225,6 @@ class Dartdoc {
     if (config.showStats) {
       logInfo(runtimeStats.buildReport());
     }
-    await packageBuilder.dispose();
     return DartdocResults(config.topLevelPackageMeta, packageGraph, _outputDir);
   }
 

lib/src/model/inheriting_container.dart

@@ -19,9 +19,10 @@ import 'package:meta/meta.dart';
 /// Note that [Constructor]s are not considered to be modifiers so a
 /// [hasModifiers] override is not necessary for this mixin.
 mixin Constructable implements InheritingContainer {
-  late final List<Constructor> constructors = element.constructors
-      .map((e) => getModelFor(e, library) as Constructor)
-      .toList(growable: false);
+  late final List<Constructor> constructors =
+      (element.augmented?.constructors ?? element.constructors)
+          .map((e) => getModelFor(e, library) as Constructor)
+          .toList(growable: false);
 
   @override
   late final List<Constructor> publicConstructorsSorted =
@@ -153,12 +154,12 @@ abstract class InheritingContainer extends Container
 
     // The mapping of all of the inherited element names to their _concrete_
     // implementation element.
-    var concreteInheritanceMap =
-        packageGraph.inheritanceManager.getInheritedConcreteMap2(element);
+    var concreteInheritanceMap = packageGraph.inheritanceManager
+        .getInheritedConcreteMap2(element.augmentedDeclarationOrSelf);
     // The mapping of all inherited element names to the nearest inherited
     // element that they resolve to.
-    var inheritanceMap =
-        packageGraph.inheritanceManager.getInheritedMap2(element);
+    var inheritanceMap = packageGraph.inheritanceManager
+        .getInheritedMap2(element.augmentedDeclarationOrSelf);
 
     var inheritanceChainElements =
         inheritanceChain.map((c) => c.element).toList(growable: false);
@@ -256,9 +257,10 @@ abstract class InheritingContainer extends Container
   }();
 
   @override
-  late final List<Method> declaredMethods = element.methods
-      .map((e) => getModelFor(e, library) as Method)
-      .toList(growable: false);
+  late final List<Method> declaredMethods =
+      (element.augmented?.methods ?? element.methods)
+          .map((e) => getModelFor(e, library) as Method)
+          .toList(growable: false);
 
   @override
   late final List<TypeParameter> typeParameters = element.typeParameters
@@ -594,6 +596,13 @@ mixin MixedInTypes on InheritingContainer {
       mixedInTypes.wherePublic;
 }
 
+extension on InterfaceElement {
+  /// This element's augmented declaration, or, if there is none, then just this
+  /// element itself.
+  InterfaceElement get augmentedDeclarationOrSelf =>
+      augmented?.declaration ?? this;
+}
+
 extension on InterfaceElement {
   bool get isDartCoreObject => name == 'Object' && library.name == 'dart.core';
 }

lib/src/model/package_builder.dart

@@ -38,8 +38,6 @@ abstract class PackageBuilder {
   // Builds package graph to be used by documentation generator.
   Future<PackageGraph> buildPackageGraph();
 
-  Future<void> dispose();
-
   /// The `include-external` option is deprecated, so we track whether it was
   /// used, to report it.
   bool get includeExternalsWasSpecified;
@@ -119,7 +117,11 @@ class PubPackageBuilder implements PackageBuilder {
 
     logDebug('${DateTime.now()}: Initializing package graph...');
     runtimeStats.startPerfTask('initializePackageGraph');
-    await newGraph.initializePackageGraph();
+    try {
+      await newGraph.initializePackageGraph();
+    } finally {
+      await _dispose();
+    }
     runtimeStats.endPerfTask();
 
     runtimeStats.startPerfTask('initializeCategories');
@@ -129,8 +131,7 @@ class PubPackageBuilder implements PackageBuilder {
     return newGraph;
   }
 
-  @override
-  Future<void> dispose() async {
+  Future<void> _dispose() async {
     // Shutdown macro support.
     await _contextCollection.dispose();
   }