Type definitions for 'Symbols as WeakMap keys' (#76) (#54195)

Signed-off-by: Leo Elmecker <lelmeckerpla@bloomberg.net>

src/harness/fourslashInterfaceImpl.ts

@@ -1225,6 +1225,8 @@ export namespace Completion {
         varEntry("Float64Array"),
         interfaceEntry("Float64ArrayConstructor"),
         moduleEntry("Intl"),
+        typeEntry("WeakKey"),
+        interfaceEntry("WeakKeyTypes"),
     ];
 
     export const globalThisEntry: ExpectedCompletionEntry = {