Support map in generated dart:html files.

client/html/src/_Collections.dart

Index: client/html/src/_Collections.dart
diff --git a/client/html/src/_Collections.dart b/client/html/src/_Collections.dart
index 3a3d3c5a0fec60453582eb2ff61d7b82c222d250..785e669fd2a927b99190edfc782d6ea6bee55d7b 100644
--- a/client/html/src/_Collections.dart
+++ b/client/html/src/_Collections.dart
@@ -14,6 +14,15 @@ class _Collections {
     }
   }
 
+  static List map(Iterable<Object> source,
+                  List<Object> destination,
+                  f(o)) {
+    for (final e in source) {
+      destination.add(f(e));
+    }
+    return destination;
+  }
+
   static bool some(Iterable<Object> iterable, bool f(Object o)) {
     for (final e in iterable) {
       if (f(e)) return true;