Fix a few strong mode errors in dart:html

tools/dom/scripts/systemhtml.py

Index: tools/dom/scripts/systemhtml.py
diff --git a/tools/dom/scripts/systemhtml.py b/tools/dom/scripts/systemhtml.py
index efceff66ac78b2067fdbd8f14cbe5329cbd014fd..2e0066a17aabb6f95fbff22d138964fe34c5d9c8 100644
--- a/tools/dom/scripts/systemhtml.py
+++ b/tools/dom/scripts/systemhtml.py
@@ -908,6 +908,15 @@ class Dart2JSBackend(HtmlDartGenerator):
       self._AddAttributeUsingProperties(attribute, html_name, read_only)
       return
 
+    # If the attribute is shadowed incompatibly in a subclass then we also
+    # can't just generate it as a field. In particular, this happens with
+    # DomMatrixReadOnly and its subclass DomMatrix. Force the superclass
+    # to generate getters. Hardcoding the known problem classes for now.
+    # TODO(alanknight): Fix this more generally.
+    if (self._interface.id == 'DOMMatrixReadOnly' or self._interface.id == 'DOMPointReadOnly'):
+        self._AddAttributeUsingProperties(attribute, html_name, read_only)
+        return
+
     # If the type has a conversion we need a getter or setter to contain the
     # conversion code.
     if (self._OutputConversion(attribute.type.id, attribute.id) or