FunctionType now has a List<NamedType> which must be sorted by name.
Previously, named parameters were stored in a Map<String, DartType>.
FunctionNode still has a List<VariableDeclaration>, but this list must
now be sorted by name.
BUG=
R=ahe@google.com
Review URL: https://codereview.chromium.org/2502343002 .