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_variadic_constraints.py

@@ -0,0 +1,74 @@
+def WebIDLTest(parser, harness):
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface VariadicConstraints1 {
+              undefined foo(byte... arg1, byte arg2);
+            };
+        """
+        )
+        results = parser.finish()
+
+    except:
+        threw = True
+
+    harness.ok(
+        threw,
+        "Should have thrown on variadic argument followed by required " "argument.",
+    )
+
+    parser = parser.reset()
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface VariadicConstraints2 {
+              undefined foo(byte... arg1, optional byte arg2);
+            };
+        """
+        )
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(
+        threw,
+        "Should have thrown on variadic argument followed by optional " "argument.",
+    )
+
+    parser = parser.reset()
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface VariadicConstraints3 {
+              undefined foo(optional byte... arg1);
+            };
+        """
+        )
+        results = parser.finish()
+
+    except:
+        threw = True
+
+    harness.ok(
+        threw,
+        "Should have thrown on variadic argument explicitly flagged as " "optional.",
+    )
+
+    parser = parser.reset()
+    threw = False
+    try:
+        parser.parse(
+            """
+            interface VariadicConstraints4 {
+              undefined foo(byte... arg1 = 0);
+            };
+        """
+        )
+        results = parser.finish()
+    except:
+        threw = True
+
+    harness.ok(threw, "Should have thrown on variadic argument with default value.")