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..dacf929e0ac2e9687b3e8e60f0961e6b7e24faad 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 K] = v as V; });

[Lasse Reichstein Nielsen, 2016/05/13 05:31:07]

I saw some other code be changed to 
  k as Object/*=K*/
for performance in spec mode. Would that make sense here too?

[Lasse Reichstein Nielsen, 2016/05/13 06:57:52]

That was an old comment I forgot to send at the time, you already fixed that.

     return result;
   }