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_usvstring.py

@@ -0,0 +1,40 @@
+# -*- coding: UTF-8 -*-
+
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+        interface TestUSVString {
+          attribute USVString svs;
+        };
+    """
+    )
+
+    results = parser.finish()
+
+    harness.check(len(results), 1, "Should be one production")
+    harness.ok(isinstance(results[0], WebIDL.IDLInterface), "Should be an IDLInterface")
+    iface = results[0]
+    harness.check(
+        iface.identifier.QName(), "::TestUSVString", "Interface has the right QName"
+    )
+    harness.check(
+        iface.identifier.name, "TestUSVString", "Interface has the right name"
+    )
+    harness.check(iface.parent, None, "Interface has no parent")
+
+    members = iface.members
+    harness.check(len(members), 1, "Should be one member")
+
+    attr = members[0]
+    harness.ok(isinstance(attr, WebIDL.IDLAttribute), "Should be an IDLAttribute")
+    harness.check(
+        attr.identifier.QName(), "::TestUSVString::svs", "Attr has correct QName"
+    )
+    harness.check(attr.identifier.name, "svs", "Attr has correct name")
+    harness.check(str(attr.type), "USVString", "Attr type is the correct name")
+    harness.ok(attr.type.isUSVString(), "Should be USVString type")
+    harness.ok(attr.type.isString(), "Should be String collective type")
+    harness.ok(not attr.type.isDOMString(), "Should be not be DOMString type")