Convert String and List to abstract classes.

lib/coreimpl/collections.dart

Index: lib/coreimpl/collections.dart
diff --git a/lib/coreimpl/collections.dart b/lib/coreimpl/collections.dart
index 07f0abad47e1bd4005746a6a55f85522b184e4a5..e9bd4e6ca5b2a43d0375bc818624c3285d0fc910 100644
--- a/lib/coreimpl/collections.dart
+++ b/lib/coreimpl/collections.dart
@@ -36,8 +36,8 @@ class Collections {
   }
 
   static Dynamic reduce(Iterable iterable,
-                        Dynamic initialValue,
-                        Dynamic combine(Dynamic previousValue, element)) {
+                        var initialValue,
+                        Dynamic combine(var previousValue, element)) {
     for (final element in iterable) {
       initialValue = combine(initialValue, element);
     }