Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

third_party/WebIDL/tests/test_empty_sequence_default_value.py

@@ -0,0 +1,54 @@
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface X {
+              const sequence<long> foo = [];
+            };
+        """
+        )
+
+        results = parser.finish()
+    except Exception as x:
+        threw = True
+
+    harness.ok(threw, "Constant cannot have [] as a default value")
+
+    parser = parser.reset()
+
+    parser.parse(
+        """
+        interface X {
+          undefined foo(optional sequence<long> arg = []);
+        };
+    """
+    )
+    results = parser.finish()
+
+    harness.ok(
+        isinstance(
+            results[0].members[0].signatures()[0][1][0].defaultValue,
+            WebIDL.IDLEmptySequenceValue,
+        ),
+        "Should have IDLEmptySequenceValue as default value of argument",
+    )
+
+    parser = parser.reset()
+
+    parser.parse(
+        """
+        dictionary X {
+          sequence<long> foo = [];
+        };
+    """
+    )
+    results = parser.finish()
+
+    harness.ok(
+        isinstance(results[0].members[0].defaultValue, WebIDL.IDLEmptySequenceValue),
+        "Should have IDLEmptySequenceValue as default value of " "dictionary member",
+    )