Make dart:collection strong-mode clean.

sdk/lib/collection/hash_map.dart

Index: sdk/lib/collection/hash_map.dart
diff --git a/sdk/lib/collection/hash_map.dart b/sdk/lib/collection/hash_map.dart
index a5f31b6fff51a824ace97e11206e0f7f4640e7b6..c8c31c529f90aba6c1aebf548f202209faa13678 100644
--- a/sdk/lib/collection/hash_map.dart
+++ b/sdk/lib/collection/hash_map.dart
@@ -102,7 +102,7 @@ abstract class HashMap<K, V> implements Map<K, V> {
    */
   factory HashMap.from(Map other) {
     HashMap<K, V> result = new HashMap<K, V>();
-    other.forEach((k, v) { result[k] = v; });
+    other.forEach((k, v) { result[k as Object/*=K*/] = v as Object/*=V*/; });
     return result;
   }