Short circuit for 'as dynamic'

tool/input_sdk/private/ddc_runtime/operations.dart

 // Copyright (c) 2015, 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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 _canonicalFieldName(obj, name, args, displayName) => JS('', '''(() => {
   $name = $canonicalMember($obj, $name);
@@ skipping 235 matching lines @@
   let result = $strongInstanceOf($obj, $type);
   if (result !== null) return result;
   let actual = $getReifiedType($obj);
   $throwStrongModeError('Strong mode is check failure: ' +
     $typeName(actual) + ' does not soundly subtype ' +
     $typeName($type));
 })()''');
 
 @JSExportName('as')
 cast(obj, type) {
-  if (obj == null) return obj;
-
+  if (JS('bool', '# == #', type, dynamic) || obj == null) return obj;

[Jennifer Messerly, 2016/05/26 18:14:51]

super trivial but: wouldn't null be more common than casts against `dynamic`?

[Leaf, 2016/05/26 18:16:37]

Maybe, but I doubt it.  By far the majority of the casts that I've measured are
against dynamic, because of our generic argument checks (e.g.
List<dynamic>.add(x) casts x to dynamic).

   bool result = strongInstanceOf(obj, type, true);
   if (JS('bool', '#', result)) return obj;
   _throwCastError(obj, type, result);
 }
 
 bool test(obj) {
   if (obj is bool) return obj;
   return booleanConversionFailed(obj);
 }
 
@@ skipping 320 matching lines @@
     constructor(dartIterator) {
       this.dartIterator = dartIterator;
     }
     next() {
       let i = this.dartIterator;
       let done = !i.moveNext();
       return { done: done, value: done ? void 0 : i.current };
     }
   }
 ''');