dart2js: use Es6 maps when available.

sdk/lib/_internal/compiler/js_lib/annotations.dart

 // Copyright (c) 2013, 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.
 
 part of _js_helper;
 
 /// Tells the optimizing compiler that the annotated method has no
-/// side-effects.
+/// side-effects. Allocations don't count as side-effects, since they can be
+/// dropped without changing the semantics of the program.
+///
 /// Requires @NoInline() to function correctly.
 class NoSideEffects {
   const NoSideEffects();
 }
 
 /// Tells the optimizing compiler that the annotated method cannot throw.
 /// Requires @NoInline() to function correctly.
 class NoThrows {
   const NoThrows();
 }
@@ skipping 28 matching lines @@
 }
 
 /// Annotation that marks the declaration as a patch.
 const _Patch patch = const _Patch(null);
 
 /// Annotation that marks the declaration as a patch for the old emitter.
 const _Patch patch_old = const _Patch('old');
 
 /// Annotation that marks the declaration as a patch for the new emitter.
 const _Patch patch_new = const _Patch('new');