Unpack buildCategories and add categoryToContainer as getter

Author: jcollins-g

lib/src/model.dart

@@ -4557,17 +4557,15 @@ class Category extends LibraryContainer implements Comparable<Category> {
 class Package extends LibraryContainer implements Comparable<Package>, Privacy {
   String _name;
   PackageGraph _packageGraph;
+
+  final Map<String, Category> _categoryToContainer = {};
   Package(this._name, this._packageGraph);
 
   /// Return true if the code has defined non-default categories for libraries
   /// in this package.
   bool get hasCategories => categories.isNotEmpty;
 
-  LibraryContainer _defaultCategory;
-  LibraryContainer get defaultCategory {
-    if (_defaultCategory == null) _buildCategories();
-    return _defaultCategory;
-  }
+  LibraryContainer get defaultCategory => categoryToContainer[null];
 
   bool _isPublic;
   @override
@@ -4587,26 +4585,28 @@ class Package extends LibraryContainer implements Comparable<Package>, Privacy {
   @override
   List<Library> get publicLibraries => super.publicLibraries;
 
-  void _buildCategories() {
-    Map<String, Category> categoryToContainer = {
-      null: new Category(null, this, dartdocOptions)
-    };
-    for (Library lib in libraries) {
-      String category = lib.category;
-      categoryToContainer.putIfAbsent(
-          category, () => new Category(category, this, dartdocOptions));
-      categoryToContainer[category]._libraries.add(lib);
+  // A map of category name to the category itself.
+  Map<String, Category> get categoryToContainer {
+    if (_categoryToContainer.isEmpty) {
+      _categoryToContainer[null] = new Category(null, this, dartdocOptions);
+      for (Library lib in libraries) {
+        String category = lib.category;
+        _categoryToContainer.putIfAbsent(
+            category, () => new Category(category, this, dartdocOptions));
+        _categoryToContainer[category]._libraries.add(lib);
+      }
     }
-    _categories = categoryToContainer.values
-        .where((c) => c.name != null)
-        .toList()
-          ..sort();
-    _defaultCategory = categoryToContainer[null];
+    return _categoryToContainer;
   }
 
   List<LibraryContainer> _categories;
   List<LibraryContainer> get categories {
-    if (_categories == null) _buildCategories();
+    if (_categories == null) {
+      _categories = categoryToContainer.values
+          .where((c) => c.name != null)
+          .toList()
+        ..sort();
+    }
     return _categories;
   }