Use "bool" as the downward inference context for assert conditions.

pkg/front_end/testcases/inference/assert_initializer.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 T f<T>() => null;
 
 class C {
-  C.expressionOnly() : assert(/*@typeArgs=dynamic*/ f());
+  C.expressionOnly() : assert(/*@typeArgs=bool*/ f());
   C.expressionAndMessage()
-      : assert(/*@typeArgs=dynamic*/ f(), /*@typeArgs=dynamic*/ f());
+      : assert(/*@typeArgs=bool*/ f(), /*@typeArgs=dynamic*/ f());
 }
 
 main() {
   // Test type inference of assert statements just to verify that the behavior
   // is the same.
-  assert(/*@typeArgs=dynamic*/ f());
-  assert(/*@typeArgs=dynamic*/ f(), /*@typeArgs=dynamic*/ f());
+  assert(/*@typeArgs=bool*/ f());
+  assert(/*@typeArgs=bool*/ f(), /*@typeArgs=dynamic*/ f());
 }