Add a define to assume ES6 is available.

PiperOrigin-RevId: 698913601

Author: Dustyn Loyda

src/com/google/javascript/jscomp/js/es6/util/iteratorfromarray.js

@@ -29,6 +29,9 @@
  * @suppress {checkTypes|reportUnknownTypes}
  */
 $jscomp.iteratorFromArray = function(array, transform) {
+  if ($jscomp.ASSUME_ES6){
+    return array[Symbol.iterator]();
+  }
   // NOTE: IE8 doesn't support indexing from boxed Strings.
   if (array instanceof String) array = array + '';
   var i = 0;

src/com/google/javascript/jscomp/js/util/defines.js

@@ -24,6 +24,13 @@
  */
 $jscomp.ASSUME_ES5 = false;
 
+/**
+ * Whether to assume ES6 is available.  This enables removing several
+ * internal polyfills, which must otherwise be detected at runtime.
+ * @define {boolean}
+ */
+$jscomp.ASSUME_ES6 = false;
+
 /**
  * Whether to skip the conformance check and simply use the polyfill always.
  * @define {boolean}