browser, worker: feed experiments dummy devices, closes #454

* just returning `None` as dummy device (like ExamineDeviceMgr)
is not explicit enough, certainly hard to debug
* introducing a special flag for the `build` action does not
seem the right place

artiq/browser/experiments.py

@@ -376,8 +376,8 @@ class ExperimentsArea(QtWidgets.QMdiArea):
         self._ddb = LocalDatasetDB(datasets_sub)
 
         self.worker_handlers = {
-            "get_device_db": lambda: None,
-            "get_device": lambda k: None,
+            "get_device_db": lambda: {},
+            "get_device": lambda k: {"type": "dummy"},
             "get_dataset": self._ddb.get,
             "update_dataset": self._ddb.update,
         }

artiq/master/worker_db.py

@@ -85,6 +85,10 @@ class RIDCounter:
         return r
 
 
+class DummyDevice:
+    pass
+
+
 def _create_device(desc, device_mgr):
     ty = desc["type"]
     if ty == "local":
@@ -109,6 +113,8 @@ def _create_device(desc, device_mgr):
         else:
             cls = Client
         return cls(controller["host"], controller["port"], desc["target_name"])
+    elif ty == "dummy":
+        return DummyDevice()
     else:
         raise ValueError("Unsupported type in device DB: " + ty)