feat(core): Implement a typed map

packages/moxxmpp/lib/src/util/typed_map.dart

@@ -0,0 +1,13 @@
+/// A map, similar to Map, but always uses the type of the value as the key.
+class TypedMap {
+  /// The internal mapping of type -> data
+  final Map<Object, Object> _data = {};
+
+  /// Associate the type of [value] with [value] in the map.
+  void set<T extends Object>(T value) {
+    _data[T] = value;
+  }
+
+  /// Return the object of type [T] from the map, if it has been stored.
+  T? get<T>() => _data[T] as T?;
+}

packages/moxxmpp/test/type_map_test.dart

@@ -0,0 +1,25 @@
+import 'package:moxxmpp/src/util/typed_map.dart';
+import 'package:test/test.dart';
+
+class TestType1 {
+  const TestType1(this.i);
+  final int i;
+}
+
+class TestType2 {
+  const TestType2(this.j);
+  final bool j;
+}
+
+void main() {
+  test('Test storing data in the type map', () {
+    // Set
+    final map = TypedMap()
+      ..set(const TestType1(1))
+      ..set(const TestType2(false));
+
+    // And access
+    expect(map.get<TestType1>()?.i, 1);
+    expect(map.get<TestType2>()?.j, false);
+  });
+}