Added tests to previously broken functionality and added null checks.

tools/dom/scripts/htmldartgenerator.py

Index: tools/dom/scripts/htmldartgenerator.py
diff --git a/tools/dom/scripts/htmldartgenerator.py b/tools/dom/scripts/htmldartgenerator.py
index 0dc105415817ab79c08e15ece11bbb821d4f9215..a60eae582fce992f077186868cb8040a7553ff6b 100644
--- a/tools/dom/scripts/htmldartgenerator.py
+++ b/tools/dom/scripts/htmldartgenerator.py
@@ -236,15 +236,22 @@ class HtmlDartGenerator(object):
 
     def GenerateChecksAndCall(signature_index, argument_count):
       checks = []
-      for i in range(0, argument_count):
+      for i in reversed(range(0, argument_count)):
         argument = signatures[signature_index][i]
         parameter_name = parameter_names[i]
         test_type = self._DartType(argument.type.id)
+
         if test_type in ['dynamic', 'Object']:
           checks.append('?%s' % parameter_name)
         elif not can_omit_type_check(test_type, i):
-          checks.append('(%s is %s || %s == null)' % (
-              parameter_name, test_type, parameter_name))
+          checks.append('%s is %s' % (
+              parameter_name, test_type))
+
+        for signature in signatures:
+          if argument not in signature:
+            checks.append('%s != null' % parameter_name)
+            break
+
       # There can be multiple presence checks.  We need them all since a later
       # optional argument could have been passed by name, leaving 'holes'.
       checks.extend(['!?%s' % name for name in parameter_names[argument_count:]])