ValueSerializer: Distinguish between 'undefined' and an absent property.

test/unittests/value-serializer-unittest.cc

Index: test/unittests/value-serializer-unittest.cc
diff --git a/test/unittests/value-serializer-unittest.cc b/test/unittests/value-serializer-unittest.cc
index db663fafd83f4882b0eaf8eec4bcd07245400ea1..ec2df077f57021ad0e12d6f4c7c68519bbcc12dd 100644
--- a/test/unittests/value-serializer-unittest.cc
+++ b/test/unittests/value-serializer-unittest.cc
@@ -1245,6 +1245,36 @@ TEST_F(ValueSerializerTest, DecodeSparseArrayVersion0) {
       });
 }
 
+TEST_F(ValueSerializerTest, RoundTripDenseArrayContainingUndefined) {
+  // In previous serialization versions, this would be interpreted as an absent
+  // property.
+  RoundTripTest("[undefined]", [this](Local<Value> value) {
+    ASSERT_TRUE(value->IsArray());
+    EXPECT_EQ(1u, Array::Cast(*value)->Length());
+    EXPECT_TRUE(EvaluateScriptForResultBool("result.hasOwnProperty(0)"));
+    EXPECT_TRUE(EvaluateScriptForResultBool("result[0] === undefined"));
+  });
+}
+
+TEST_F(ValueSerializerTest, DecodeDenseArrayContainingUndefined) {
+  // In previous versions, "undefined" in a dense array signified absence of the
+  // element (for compatibility). In new versions, it has a separate encoding.
+  DecodeTest({0xff, 0x09, 0x41, 0x01, 0x5f, 0x24, 0x00, 0x01},
+             [this](Local<Value> value) {
+               EXPECT_TRUE(EvaluateScriptForResultBool("!(0 in result)"));
+             });
+  DecodeTest(
+      {0xff, 0x0b, 0x41, 0x01, 0x5f, 0x24, 0x00, 0x01},
+      [this](Local<Value> value) {
+        EXPECT_TRUE(EvaluateScriptForResultBool("0 in result"));
+        EXPECT_TRUE(EvaluateScriptForResultBool("result[0] === undefined"));
+      });
+  DecodeTest({0xff, 0x0b, 0x41, 0x01, 0x2d, 0x24, 0x00, 0x01},
+             [this](Local<Value> value) {
+               EXPECT_TRUE(EvaluateScriptForResultBool("!(0 in result)"));
+             });
+}
+
 TEST_F(ValueSerializerTest, RoundTripDate) {
   RoundTripTest("new Date(1e6)", [](Local<Value> value) {
     ASSERT_TRUE(value->IsDate());