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_optional_constraints.py

@@ -0,0 +1,35 @@
+def WebIDLTest(parser, harness):
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface OptionalConstraints1 {
+              undefined foo(optional byte arg1, byte arg2);
+            };
+        """
+        )
+
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        not threw,
+        "Should not have thrown on non-optional argument following "
+        "optional argument.",
+    )
+
+    parser = parser.reset()
+    parser.parse(
+        """
+        interface OptionalConstraints2 {
+          undefined foo(optional byte arg1 = 1, optional byte arg2 = 2,
+                        optional byte arg3, optional byte arg4 = 4,
+                        optional byte arg5, optional byte arg6 = 9);
+        };
+    """
+    )
+    results = parser.finish()
+    args = results[0].members[0].signatures()[0][1]
+    harness.check(len(args), 6, "Should have 6 arguments")
+    harness.check(args[5].defaultValue.value, 9, "Should have correct default value")