ES6: Make sure we do not store -0 as the key in Map/Set

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index 20887dd8f7adf5f4ae61c0919c8afbd09469e341..0027bd732041f94ccf81dbfe3256c4f75d680c5a 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -49,6 +49,13 @@ function SetAddJS(key) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Set.prototype.add', this]);
   }
+  // Normalize -0 to +0 as required by the spec.
+  // Even though we use SameValueZero as the comparison for the keys we don't
+  // want to ever store -0 as the key since the key is directly exposed when
+  // doing iteration.
+  if (key === 0) {
+    key = 0;
+  }
   return %SetAdd(this, key);
 }
 
@@ -186,6 +193,13 @@ function MapSetJS(key, value) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['Map.prototype.set', this]);
   }
+  // Normalize -0 to +0 as required by the spec.
+  // Even though we use SameValueZero as the comparison for the keys we don't
+  // want to ever store -0 as the key since the key is directly exposed when
+  // doing iteration.
+  if (key === 0) {
+    key = 0;
+  }
   return %MapSet(this, key, value);
 }