Turn on playground for "obj ?= 2"

Without it fail with different issue, doesn’t connected to real case:
`Got error message: Unexpected token (1:5)`.

test/tests-6to5-playground.js

@@ -197,7 +197,9 @@ test("var foo = { memo bar() {} };",
 
 // Memoization assignment operator
 
-testFail("obj ?= 2;", "You can only use member expressions in memoization assignment (1:0)");
+testFail("obj ?= 2;", "You can only use member expressions in memoization assignment (1:0)", {
+  playground: true
+});
 
 test("obj.x ?= 2;", {
   type: "Program",