Use for-of loops in collection constructors

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index 8d2d18428602b85979ff98afde6f60ea2c30ff92..c8cf639f97d96c3396f463a97ad5181ac4ccc347 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -20,26 +20,17 @@ function SetConstructor(iterable) {
     throw MakeTypeError('constructor_not_function', ['Set']);
   }
 
-  var iter, adder;
+  %_SetInitialize(this);
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    adder = this.add;
+    var adder = this.add;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['add', this]);
     }
-    iter = GetIterator(iterable);
-  }
-
-  %_SetInitialize(this);
-
-  if (IS_UNDEFINED(iter)) return;
 
-  var next, done;
-  while (!(next = iter.next()).done) {
-    if (!IS_SPEC_OBJECT(next)) {
-      throw MakeTypeError('iterator_result_not_an_object', [next]);
+    for (var value of iterable) {
+      %_CallFunction(this, value, adder);
     }
-    %_CallFunction(this, next.value, adder);
   }
 }
 
@@ -160,30 +151,20 @@ function MapConstructor(iterable) {
     throw MakeTypeError('constructor_not_function', ['Map']);
   }
 
-  var iter, adder;
+  %_MapInitialize(this);
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    adder = this.set;
+    var adder = this.set;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['set', this]);
     }
-    iter = GetIterator(iterable);
-  }
-
-  %_MapInitialize(this);
 
-  if (IS_UNDEFINED(iter)) return;
-
-  var next, done, nextItem;
-  while (!(next = iter.next()).done) {
-    if (!IS_SPEC_OBJECT(next)) {
-      throw MakeTypeError('iterator_result_not_an_object', [next]);
-    }
-    nextItem = next.value;
-    if (!IS_SPEC_OBJECT(nextItem)) {
-      throw MakeTypeError('iterator_value_not_an_object', [nextItem]);
+    for (var nextItem of iterable) {
+      if (!IS_SPEC_OBJECT(nextItem)) {
+        throw MakeTypeError('iterator_value_not_an_object', [nextItem]);
+      }
+      %_CallFunction(this, nextItem[0], nextItem[1], adder);
     }
-    %_CallFunction(this, nextItem[0], nextItem[1], adder);
   }
 }