Add a regression test for #2090 (#2281)

Summary:
https://github.com/facebook/prepack/issues/2090 doesn't reproduce for me anymore so I'm adding a test.
Pull Request resolved: https://github.com/facebook/prepack/pull/2281

Differential Revision: D8893196

Pulled By: gaearon

fbshipit-source-id: c0d9748e5898ab8cffa0c6c23dd5db9dadfbf6cc

test/serializer/additional-functions/RegressionTestForIssue2090.js

@@ -0,0 +1,35 @@
+(function() {
+  function fn(arg) {
+    if (arg.foo) {
+      return "a";
+    }
+    if (arg.bar === 0) {
+      return "b";
+    }
+    return Object.assign(
+      {
+        qux: function() {
+          arg.qux();
+        },
+      },
+      arg.baz()
+    );
+  }
+
+  if (global.__optimize) {
+    __optimize(fn);
+  }
+
+  global.inspect = function() {
+    return JSON.stringify([
+      fn({ foo: 1 }),
+      fn({ bar: 0 }),
+      fn({ foo: 1, bar: 0 }),
+      fn({
+        baz: function() {
+          return { a: 42 };
+        },
+      }),
+    ]);
+  };
+})();