Add supported checks to the SVG library, and library cleanup.

tools/dom/scripts/monitored.py

Index: tools/dom/scripts/monitored.py
diff --git a/tools/dom/scripts/monitored.py b/tools/dom/scripts/monitored.py
index aa26502268555ea6a6c275c69b25e515db9bae9e..e90330ae7c6ae66cfe2c727af35871efbfb1cd1f 100644
--- a/tools/dom/scripts/monitored.py
+++ b/tools/dom/scripts/monitored.py
@@ -29,6 +29,9 @@ class Dict(MonitoredCollection):
     self._used_keys.add(key)
     return self._map[key]
 
+  def __setitem__(self, key, value):
+    self._map[key] = value
+
   def __contains__(self, key):
     self._used_keys.add(key)
     return key in self._map