[relay] Fix RecordSourceInspector not being usable as class.

types/relay-runtime/index.d.ts

@@ -843,7 +843,7 @@ export class RelayRecordSourceInspector {
 }
 
 // note RecordSourceInspector is only available in dev environment
-export type RecordSourceInspector = RelayRecordSourceInspector;
+export class RecordSourceInspector extends RelayRecordSourceInspector {}
 
 // ~~~~~~~~~~~~~~~~~~~~~
 // RelayObservable

types/relay-runtime/relay-runtime-tests.tsx

@@ -1,4 +1,4 @@
-import { Environment, Network, RecordSource, Store, ConnectionHandler, ViewerHandler } from "relay-runtime";
+import { Environment, Network, RecordSource, Store, ConnectionHandler, ViewerHandler, RecordSourceInspector } from "relay-runtime";
 
 const source = new RecordSource();
 const store = new Store(source);
@@ -46,3 +46,9 @@ function handlerProvider(handle: any) {
     }
     throw new Error(`handlerProvider: No handler provided for ${handle}`);
 }
+
+// ~~~~~~~~~~~~~~~~~~~~~
+// Source
+// ~~~~~~~~~~~~~~~~~~~~~
+
+const inspector = new RecordSourceInspector(source);