can not work in dart 2, flutter beta3

[badboy-tian]

compiler message: file:///Users/tian/.pub-cache/hosted/pub.dartlang.org/quiver_collection-1.0.0/lib/src/multimap.dart: Error: The parameter 'initialValue' of the method '_WrappedList::fold' has type quiver.collection::_WrappedList::fold::T, which does not match the corresponding type in the overridden method (dynamic).
compiler message: Change to a supertype of dynamic (or, for a covariant parameter, a subtype).
compiler message: file:///Users/tian/.pub-cache/hosted/pub.dartlang.org/quiver_collection-1.0.0/lib/src/multimap.dart:315:3: Context: This is the overriden method ('fold').
compiler message:   fold(initialValue, combine(previousValue, V element)) {
compiler message:   ^