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_union_nullable.py

@@ -0,0 +1,60 @@
+def WebIDLTest(parser, harness):
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface OneNullableInUnion {
+              undefined foo((object? or DOMString?) arg);
+            };
+        """
+        )
+
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(threw, "Two nullable member types of a union should have thrown.")
+
+    parser.reset()
+    threw = False
+
+    try:
+        parser.parse(
+            """
+            interface NullableInNullableUnion {
+              undefined foo((object? or DOMString)? arg);
+            };
+        """
+        )
+
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        threw,
+        "A nullable union type with a nullable member type should have " "thrown.",
+    )
+
+    parser.reset()
+    threw = False
+
+    try:
+        parser.parse(
+            """
+            interface NullableInUnionNullableUnionHelper {
+            };
+            interface NullableInUnionNullableUnion {
+              undefined foo(((object? or DOMString) or NullableInUnionNullableUnionHelper)? arg);
+            };
+        """
+        )
+
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        threw,
+        "A nullable union type with a nullable member type should have " "thrown.",
+    )