Warnings for instantiating an abstract type.

frog/utils.dart

Index: frog/utils.dart
diff --git a/frog/utils.dart b/frog/utils.dart
index 657349a6cf0c7d20ca3ed3c89286791ad5b32128..cb3ca2627c0a682b78a71af7f8d99abf6f1270f7 100644
--- a/frog/utils.dart
+++ b/frog/utils.dart
@@ -7,8 +7,8 @@
 
 // TODO(jmesserly): we might want a version of this that return an iterable,
 // however JS, Python and Ruby versions are all eager.
-List map(Iterable source, mapper(source)) {
-  List result = new List();
+List map(Iterable source, Dynamic mapper(source)) {
+  List result = [];
   if (source is List) {
     List list = source; // TODO: shouldn't need this
     result.length = list.length;
@@ -36,8 +36,8 @@ reduce(Iterable source, callback, [initialValue]) {
   return current;
 }
 
-List zip(Iterable left, Iterable right, mapper(left, right)) {
-  List result = new List();
+List zip(Iterable left, Iterable right, Dynamic mapper(left, right)) {
+  List result = [];
   var x = left.iterator();
   var y = right.iterator();
   while (x.hasNext() && y.hasNext()) {
@@ -49,6 +49,16 @@ List zip(Iterable left, Iterable right, mapper(left, right)) {
   return result;
 }
 
+Map<Dynamic, List> groupBy(Iterable left, Dynamic keyFn(value)) {
+  Map result = {};
+  var iter = left.iterator();
+  while (iter.hasNext()) {
+    var i = iter.next();
+    result.putIfAbsent(keyFn(i), () => []).add(i);
+  }
+  return result;
+}
+
 /** Sorts the map by the key. */
 List orderValuesByKeys(Map map) {
   // TODO(jmesserly): it'd be nice to have SortedMap in corelib.