Backout rev 0c9ff69d4263 to re-land with proper credit. Bug 904723, no_r=me.

js/src/builtin/Array.js

@@ -660,7 +660,7 @@ function ArrayKeys() {
     return CreateArrayIterator(this, ITEM_KIND_KEY);
 }
 
-/* ES6 rev 25 (2014 May 22) 22.1.2.1 */
+/* ES6 rev 24 (2014 April 27) 22.1.2.1 */
 function ArrayFrom(arrayLike, mapfn=undefined, thisArg=undefined) {
     // Step 1.
     var C = this;
@@ -677,36 +677,23 @@ function ArrayFrom(arrayLike, mapfn=undefined, thisArg=undefined) {
     var attrs = ATTR_CONFIGURABLE | ATTR_ENUMERABLE | ATTR_WRITABLE;
 
     // Steps 6-8.
-    var usingIterator = items["@@iterator"];
-    if (usingIterator !== undefined) {
+    if (items["@@iterator"] !== undefined) {
         // Steps 8.a-c.
         var A = IsConstructor(C) ? new C() : [];
 
-        // Steps 8.d-e.
-        var iterator = callFunction(usingIterator, items);
-
         // Step 8.f.
         var k = 0;
 
-        // Steps 8.g.i-vi.
-        // These steps cannot be implemented using a for-of loop.
-        // See <https://bugs.ecmascript.org/show_bug.cgi?id=2883>.
-        var next;
-        while (true) {
-            // Steps 8.g.ii-vi.
-            next = iterator.next();
-            if (!IsObject(next))
-                ThrowError(JSMSG_NEXT_RETURNED_PRIMITIVE);
-            if (next.done)
-                break;  // Substeps of 8.g.iv are implemented below.
-            var nextValue = next.value;
-
+        // Steps 8.d-e and 8.g.i-vi.
+        for (var nextValue of items) {
             // Steps 8.g.vii-viii.
             var mappedValue = mapping ? callFunction(mapfn, thisArg, nextValue, k) : nextValue;
 
             // Steps 8.g.ix-xi.
             _DefineDataProperty(A, k++, mappedValue, attrs);
         }
+
+        // Here we're at step 8.g.iv.1. Fall through; it's implemented below.
     } else {
         // Step 9 is an assertion: items is not an Iterator. Testing this is
         // literally the very last thing we did, so we don't assert here.

js/src/js.msg

@@ -357,7 +357,7 @@ MSG_DEF(JSMSG_ALREADY_HAS_PRAGMA, 303, 2, JSEXN_ERR,      "{0} is being assigned
 MSG_DEF(JSMSG_PAR_ARRAY_BAD_ARG,      304, 0, JSEXN_RANGEERR, "invalid parallel method argument")
 MSG_DEF(JSMSG_REGEXP_RUNTIME_ERROR,   305, 0, JSEXN_INTERNALERR, "an error occurred while executing regular expression")
 MSG_DEF(JSMSG_DEBUG_OPTIMIZED_OUT,    306, 0, JSEXN_ERR, "variable has been optimized out")
-MSG_DEF(JSMSG_NEXT_RETURNED_PRIMITIVE,307, 0, JSEXN_TYPEERR, "iterator.next() returned a non-object value")
+MSG_DEF(JSMSG_UNUSED307,              307, 0, JSEXN_NONE, "")
 MSG_DEF(JSMSG_PAR_ARRAY_SCATTER_CONFLICT, 308, 0, JSEXN_ERR, "no conflict resolution function provided")
 MSG_DEF(JSMSG_PAR_ARRAY_SCATTER_BOUNDS, 309, 0, JSEXN_ERR, "index in scatter vector out of bounds")
 MSG_DEF(JSMSG_CANT_REPORT_NC_AS_NE,   310, 0, JSEXN_TYPEERR, "proxy can't report a non-configurable own property as non-existent")

js/src/tests/ecma_6/Array/from_errors.js

@@ -130,16 +130,5 @@ assertThrowsValue(() => Array.from.call(C, arrayish, () => { throw exc; }), exc)
 assertEq(log, "lC0");
 assertEq(obj instanceof C, true);
 
-// It's a TypeError if the iterator's .next() method returns a primitive.
-for (var primitive of [undefined, null, 17]) {
-    assertThrowsInstanceOf(
-        () => Array.from({
-            "@@iterator": () => {
-                next: () => primitive
-            }
-        }),
-        TypeError);
-}
-
 if (typeof reportCompare === 'function')
     reportCompare(0, 0);

js/src/tests/ecma_6/Array/from_proxy.js

@@ -36,17 +36,9 @@ assertDeepEq(log, ["define 0", "define 1", "define 2", "set length"]);
 // calls handler.get on it.
 log = [];
 assertDeepEq(Array.from(new LoggingProxy([3, 4, 5])), [3, 4, 5]);
-assertDeepEq(log, ["get @@iterator",
+assertDeepEq(log, ["get @@iterator", "get @@iterator",
                    "get length", "get 0", "get length", "get 1", "get length", "get 2",
                    "get length"]);
 
-// Array-like iteration only gets the length once.
-log = [];
-var arr = [5, 6, 7];
-arr["@@iterator"] = undefined;
-assertDeepEq(Array.from(new LoggingProxy(arr)), [5, 6, 7]);
-assertDeepEq(log, ["get @@iterator",
-                   "get length", "get 0", "get 1", "get 2"]);
-
 if (typeof reportCompare === 'function')
     reportCompare(0, 0);