Fix type warning in platform_impl.dart.

sdk/lib/io/platform_impl.dart

Index: sdk/lib/io/platform_impl.dart
diff --git a/sdk/lib/io/platform_impl.dart b/sdk/lib/io/platform_impl.dart
index cbc8443dce3773fe450e9734e8b479e30f366d81..8b858c9873a5e4069e200430c47ebb71b5212bb8 100644
--- a/sdk/lib/io/platform_impl.dart
+++ b/sdk/lib/io/platform_impl.dart
@@ -83,8 +83,8 @@ class _CaseInsensitiveStringMap<V> implements Map<String, V> {
   V remove(String key) => _map.remove(key.toUpperCase());
   void clear() => _map.clear();
   void forEach(void f(String key, V value)) => _map.forEach(f);
-  Collection<String> get keys => _map.keys;
-  Collection<V> get values => _map.values;
+  Iterable<String> get keys => _map.keys;
+  Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;