feat(viewPort): adds initial implementation of ViewPort.

ViewPort is the mechanism backing @Template directives. Those
directives can use the viewport to dynamically create, attach and
detach views.

modules/test_lib/src/test_lib.dart

@@ -33,6 +33,10 @@ class NotExpect extends gns.NotExpect {
   void toEqual(expected) => toHaveSameProps(expected);
 }
 
+beforeEach(fn) {
+  gns.beforeEach(_enableReflection(fn));
+}
+
 it(name, fn) {
   gns.it(name, _enableReflection(_handleAsync(fn)));
 }
@@ -62,4 +66,4 @@ _handleAsync(fn) {
   }
 
   return fn;
-}
+}