Do not throw if the array is empty in Map constructor

Bug: chromium:837939
Change-Id: Iaca2bc5b52f47d8add13ed9b82497a53cb522933
Reviewed-on: https://chromium-review.googlesource.com/1034043Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52913}

src/builtins/builtins-collections-gen.cc

@@ -237,6 +237,7 @@ void BaseCollectionsAssembler::AddConstructorEntriesFromFastJSArray(
   TNode<Map> original_fast_js_array_map = LoadMap(fast_jsarray);
 #endif
   Label exit(this), if_doubles(this), if_smiorobjects(this);
+  GotoIf(IntPtrEqual(length, IntPtrConstant(0)), &exit);
   Branch(IsFastSmiOrTaggedElementsKind(elements_kind), &if_smiorobjects,
          &if_doubles);
   BIND(&if_smiorobjects);

test/mjsunit/es6/regress/regress-crbug-837939.js

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+// Create a double elements array.
+const iterable = [123.123];
+assertTrue(%HasDoubleElements(iterable))
+
+iterable.length = 0;
+assertTrue(%HasDoubleElements(iterable))
+
+// Should not throw here.
+let map = new Map(iterable);
+assertEquals(0, map.size);
+new WeakMap(iterable); // WeakMap does not have a size