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..1032f706ad2d6b50d953b1c733016326577f4943 100644
--- a/tools/dom/scripts/htmldartgenerator.py
+++ b/tools/dom/scripts/htmldartgenerator.py
@@ -240,11 +240,21 @@ class HtmlDartGenerator(object):
         argument = signatures[signature_index][i]
         parameter_name = parameter_names[i]
         test_type = self._DartType(argument.type.id)
+        nullable = False or argument.type.nullable
+
         if test_type in ['dynamic', 'Object']:
           checks.append('?%s' % parameter_name)
         elif not can_omit_type_check(test_type, i):
+          nullable = True
           checks.append('(%s is %s || %s == null)' % (
               parameter_name, test_type, parameter_name))
+
+        if not nullable:
+          for signature in signatures:
+            if (len(signature) <= i or signature[i] != argument):
+              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:]])