[vm/corelib] Fix ConcurrentModificationError for empty addAll.

Since we only add elements if the iterator is non-empty, we
shouldn't get a ConcurrentModificationError if both the iterator
and receiver are the same empty EfficientLengthIterator.

Fixes #42011

TEST=New regression test added that fails without the sdk change.

Fixed: 42011
Change-Id: Ib5259c8f043cc8277d2fea407d1f64a24430c4d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173901
Reviewed-by: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

sdk/lib/_internal/vm/lib/growable_array.dart

@@ -211,6 +211,9 @@ class _GrowableList<T> extends ListBase<T> {
       var cap = _capacity;
       // Pregrow if we know iterable.length.
       var iterLen = iterable.length;
+      if (iterLen == 0) {
+        return;
+      }
       var newLen = len + iterLen;
       if (newLen > cap) {
         do {

tests/corelib/regress_42011_test.dart

@@ -0,0 +1,10 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/42011
+main() {
+  var a = [];
+  // No elements are added, so should not get a ConcurrentModificationError.
+  a.addAll(a);
+}

tests/corelib_2/regress_42011_test.dart

@@ -0,0 +1,10 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/42011
+main() {
+  var a = [];
+  // No elements are added, so should not get a ConcurrentModificationError.
+  a.addAll(a);
+}