Revert "Remove Object.dynamic."

samples/ui_lib/util/CollectionUtils.dart

Index: samples/ui_lib/util/CollectionUtils.dart
diff --git a/samples/ui_lib/util/CollectionUtils.dart b/samples/ui_lib/util/CollectionUtils.dart
index 8d633ea06f005f7c4196f2cb722359a6157d686e..4062212c3104da7086816eda23e8669c068e9cab 100644
--- a/samples/ui_lib/util/CollectionUtils.dart
+++ b/samples/ui_lib/util/CollectionUtils.dart
@@ -46,7 +46,7 @@ class CollectionUtils {
   static List map(Iterable source, var mapper) {
     // TODO(jmesserly): I was trying to set the capacity here, but instead it
     // seems to create a fixed list. Hence assigning by index below.
-    List result = new List(source is List ? (source as List).length : null);
+    List result = new List(source is List ? source.dynamic.length : null);
     int i = 0;
     for (final item in source) {
       result[i++] = mapper(item);