Enable the Body alias for RigidBody

Author: sherm1

multibody/tree/rigid_body.h

@@ -861,10 +861,10 @@ class RigidBody : public MultibodyElement<T> {
   systems::NumericParameterIndex spatial_inertia_parameter_index_;
 };
 
-// TODO(sherm1) Uncomment this alias for backwards compatibility after
-//  converting Drake to use RigidBody exclusively.
-// template <typename T>
-// using Body = RigidBody<T>;
+/// (Compatibility) Prefer RigidBody to Body, however this dispreferred alias
+/// is available to permit older code to continue working.
+template <typename T>
+using Body = RigidBody<T>;
 
 }  // namespace multibody
 }  // namespace drake

multibody/tree/test/multibody_tree_creation_test.cc

@@ -67,6 +67,11 @@ GTEST_TEST(MultibodyTree, BasicAPIToAddBodiesAndJoints) {
 
   // Retrieves the world body.
   const RigidBody<double>& world_body = model->world_body();
+  EXPECT_EQ(world_body.name(), "world");
+
+  // Make sure the (dispreferred) Body alias is working.
+  const Body<double>& also_world_body = model->world_body();
+  EXPECT_EQ(also_world_body.name(), "world");
 
   // Creates a NaN SpatialInertia to instantiate the RigidBody links of the
   // pendulum. Using a NaN spatial inertia is ok so far since we are still