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_callback_constructor.py

@@ -0,0 +1,84 @@
+import WebIDL
+
+
+def WebIDLTest(parser, harness):
+    parser.parse(
+        """
+        interface TestCallbackConstructor {
+          attribute CallbackConstructorType? constructorAttribute;
+        };
+
+        callback constructor CallbackConstructorType = TestCallbackConstructor (unsigned long arg);
+    """
+    )
+
+    results = parser.finish()
+
+    harness.ok(True, "TestCallbackConstructor interface parsed without error.")
+    harness.check(len(results), 2, "Should be two productions.")
+    iface = results[0]
+    harness.ok(isinstance(iface, WebIDL.IDLInterface), "Should be an IDLInterface")
+    harness.check(
+        iface.identifier.QName(),
+        "::TestCallbackConstructor",
+        "Interface has the right QName",
+    )
+    harness.check(
+        iface.identifier.name, "TestCallbackConstructor", "Interface has the right name"
+    )
+    harness.check(len(iface.members), 1, "Expect %s members" % 1)
+
+    attr = iface.members[0]
+    harness.ok(isinstance(attr, WebIDL.IDLAttribute), "Should be an IDLAttribute")
+    harness.ok(attr.isAttr(), "Should be an attribute")
+    harness.ok(not attr.isMethod(), "Attr is not an method")
+    harness.ok(not attr.isConst(), "Attr is not a const")
+    harness.check(
+        attr.identifier.QName(),
+        "::TestCallbackConstructor::constructorAttribute",
+        "Attr has the right QName",
+    )
+    harness.check(
+        attr.identifier.name, "constructorAttribute", "Attr has the right name"
+    )
+    t = attr.type
+    harness.ok(not isinstance(t, WebIDL.IDLWrapperType), "Attr has the right type")
+    harness.ok(isinstance(t, WebIDL.IDLNullableType), "Attr has the right type")
+    harness.ok(t.isCallback(), "Attr has the right type")
+
+    callback = results[1]
+    harness.ok(callback.isConstructor(), "Callback is constructor")
+
+    parser.reset()
+    threw = False
+    try:
+        parser.parse(
+            """
+            [LegacyTreatNonObjectAsNull]
+            callback constructor CallbackConstructorType = object ();
+        """
+        )
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        threw, "Should throw on LegacyTreatNonObjectAsNull callback constructors"
+    )
+
+    parser.reset()
+    threw = False
+    try:
+        parser.parse(
+            """
+            [MOZ_CAN_RUN_SCRIPT_BOUNDARY]
+            callback constructor CallbackConstructorType = object ();
+        """
+        )
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        threw, "Should not permit MOZ_CAN_RUN_SCRIPT_BOUNDARY callback constructors"
+    )