[polymer] Merge of master into polymer10-migration

tracing/tracing/base/iteration_helpers.html

Index: tracing/tracing/base/iteration_helpers.html
diff --git a/tracing/tracing/base/iteration_helpers.html b/tracing/tracing/base/iteration_helpers.html
index ddd3a90d43eb469dc266cd3af092138b7cf566b3..09c6519ff25b60e96cb9573caee446fd7447c0b5 100644
--- a/tracing/tracing/base/iteration_helpers.html
+++ b/tracing/tracing/base/iteration_helpers.html
@@ -11,9 +11,10 @@ found in the LICENSE file.
 'use strict';
 
 tr.exportTo('tr.b', function() {
+
   /**
    * Converts any object which is either (a) an iterable, or (b) an
-   * "array-ish" obect (has length property and can be indexed into)
+   * "array-ish" object (has length property and can be indexed into)
    * into an array.
    */
   function asArray(x) {
@@ -173,21 +174,21 @@ tr.exportTo('tr.b', function() {
   /**
    * Returns a new dictionary with items grouped by the return value of the
    * specified function being called on each item.
-   * @param {!Array.<Object>} ary The array being iterated through
-   * @param {!Function} fn The mapping function between the array value and the
-   * map key.
+   * @param {!Array.<!*>} ary The array being iterated through
+   * @param {!function(!*):!*} callback The mapping function between the array
+   * value and the map key.
+   * @param {*=} opt_this
    */
-  function group(ary, fn) {
-    return ary.reduce(function(accumulator, curr) {
-      var key = fn(curr);
-
-      if (key in accumulator)
-        accumulator[key].push(curr);
+  function group(ary, callback, opt_this) {
+    var results = {};
+    for (var element of ary) {
+      var key = callback.call(opt_this, element);
+      if (key in results)
+        results[key].push(element);
       else
-        accumulator[key] = [curr];
-
-      return accumulator;
-    }, {});
+        results[key] = [element];
+    }
+    return results;
   }
 
   function iterItems(dict, fn, opt_this) {