Return the value to which the specified key is mapped, or
the default value if this map contains no mapping for the key.
DEPRECATED: lookupDefault is deprecated as of version 0.2.11, replaced
by findWithDefault.
Looks for an icon in the list of default icon factories.
For display to the user, you should use styleLookupIconSet on
the Style for the widget that will display the icon, instead of
using this function directly, so that themes are taken into account.