Adding types to dart:html's dartium files where possible.

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index ddade576d7f903d6cf46f0ccc3b673b83c5e90a6..4258d8143b1ebc42a4cb7b97786e6daad5c55ab5 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -428,18 +428,16 @@ class DartiumBackend(HtmlDartGenerator):
       info: An OperationInfo object.
     """
 
-    operation = info.operations[0]
-
-    is_custom = 'Custom' in operation.ext_attrs
-    has_optional_arguments = any(self._IsArgumentOptionalInWebCore(operation, argument) for argument in operation.arguments)
-    needs_dispatcher = not is_custom and (len(info.operations) > 1 or has_optional_arguments)
-
     dart_declaration = '%s%s %s(%s)' % (
         'static ' if info.IsStatic() else '',
         self.SecureOutputType(info.type_name),
         html_name,
-        info.ParametersDeclaration(
-            (lambda x: 'dynamic') if needs_dispatcher else self._DartType))
+        info.ParametersDeclaration(self._DartType))
+
+    operation = info.operations[0]
+    is_custom = 'Custom' in operation.ext_attrs
+    has_optional_arguments = any(self._IsArgumentOptionalInWebCore(operation, argument) for argument in operation.arguments)
+    needs_dispatcher = not is_custom and (len(info.operations) > 1 or has_optional_arguments)
 
     if not needs_dispatcher:
       # Bind directly to native implementation