Fix a few strong mode errors in dart:html

tools/dom/scripts/systemhtml.py

 #!/usr/bin/python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 import logging
@@ skipping 890 matching lines @@
               '  // Use implementation from $SUPER.\n'
               '  // final $TYPE $NAME;\n',
               SUPER=super_attribute_interface,
               NAME=html_name,
               TYPE=self.SecureOutputType(attribute.type.id, False, read_only))
           return
       self._members_emitter.Emit('\n  // Shadowing definition.')
       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
         self._InputConversion(attribute.type.id, attribute.id)):
       self._AddAttributeUsingProperties(attribute, html_name, read_only)
       return
 
     output_type = self.SecureOutputType(attribute.type.id, False, read_only)
     input_type = self._NarrowInputType(attribute.type.id)
     metadata = self._Metadata(attribute.type.id, attribute.id, output_type)
@@ skipping 378 matching lines @@
 
   def AddFile(self, basename, library_name, path):
     self._libraries[library_name].AddFile(path)
 
   def AddTypeEntry(self, library_name, idl_name, dart_name):
     self._libraries[library_name].AddTypeEntry(idl_name, dart_name)
 
   def Emit(self, emitter, auxiliary_dir):
     for lib in self._libraries.values():
       lib.Emit(emitter, auxiliary_dir)