Add support for lazy transformers.

pkg/barback/lib/src/utils.dart

Index: pkg/barback/lib/src/utils.dart
diff --git a/pkg/barback/lib/src/utils.dart b/pkg/barback/lib/src/utils.dart
index f1bcddde7f45085a1e977c5c1d5b03ea1653f07f..e4fe08269f8354c08ad07b5d101a9cb4bc5a526a 100644
--- a/pkg/barback/lib/src/utils.dart
+++ b/pkg/barback/lib/src/utils.dart
@@ -140,10 +140,34 @@ List flatten(Iterable nested) {
 Set unionAll(Iterable<Set> sets) =>
   sets.fold(new Set(), (union, set) => union.union(set));
 
-/// Passes each key/value pair in [map] to [fn] and returns a new [Map] whose
-/// values are the return values of [fn].
-Map mapMapValues(Map map, fn(key, value)) =>
-  new Map.fromIterable(map.keys, value: (key) => fn(key, map[key]));
+/// Creates a map from [iter], using the return values of [keyFn] as the keys
+/// and the return values of [valueFn] as the values.
+Map listToMap(Iterable iter, keyFn(element), valueFn(element)) {

[Bob Nystrom, 2014/01/30 19:33:44]

Is this any different from new Map.fromIterable()?

[nweiz, 2014/01/31 03:43:27]

It's not. Removed.

+  var map = new Map();
+  for (var element in iter) {
+    map[keyFn(element)] = valueFn(element);
+  }
+  return map;
+}
+
+/// Creates a new map from [map] with new keys and values.
+///
+/// The return values of [keyFn] are used as the keys and the return values of
+/// [valueFn] are used as the values for the new map.
+Map mapMap(Map map, keyFn(key, value), valueFn(key, value)) =>
+  listToMap(map.keys,
+      (key) => keyFn(key, map[key]),
+      (key) => valueFn(key, map[key]));
+
+/// Creates a new map from [map] with the same keys.
+///
+/// The return values of [fn] are used as the values for the new map.
+Map mapMapValues(Map map, fn(key, value)) => mapMap(map, (key, _) => key, fn);
+
+/// Creates a new map from [map] with the same keys.
+///
+/// The return values of [fn] are used as the keys for the new map.
+Map mapMapKeys(Map map, fn(key, value)) => mapMap(map, fn, (_, value) => value);

[Bob Nystrom, 2014/01/30 19:33:44]

Not in this patch, but we should look at moving these into quiver.

[nweiz, 2014/01/31 03:43:27]

Absolutely.

 
 /// Returns whether [set1] has exactly the same elements as [set2].
 bool setEquals(Set set1, Set set2) =>