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

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.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.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 101 matching lines @@
   KernelAssertStatement(Expression condition,
       {Expression message, int conditionStartOffset, int conditionEndOffset})
       : super(condition,
             message: message,
             conditionStartOffset: conditionStartOffset,
             conditionEndOffset: conditionEndOffset);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
     inferrer.listener.assertStatementEnter(this);
-    inferrer.inferExpression(condition, null, false);
+    inferrer.inferExpression(
+        condition, inferrer.coreTypes.boolClass.rawType, false);
     if (message != null) {
       inferrer.inferExpression(message, null, false);
     }
     inferrer.listener.assertStatementExit(this);
   }
 }
 
 /// Shadow object for [AwaitExpression].
 class KernelAwaitExpression extends AwaitExpression
     implements KernelExpression {
@@ skipping 2328 matching lines @@
   accept(v) => unsupported("accept", -1, null);
 
   accept1(v, arg) => unsupported("accept1", -1, null);
 
   getStaticType(types) => unsupported("getStaticType", -1, null);
 
   transformChildren(v) => unsupported("transformChildren", -1, null);
 
   visitChildren(v) => unsupported("visitChildren", -1, null);
 }