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

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 5178 matching lines @@
   Object visitAsExpression(AsExpression node) {
     super.visitAsExpression(node);
     // Since an as-statement doesn't actually change the type, we don't
     // let it affect the propagated type when it would result in a loss
     // of precision.
     overrideExpression(node.expression, node.type.type, false, false);
     return null;
   }
 
   @override
+  Object visitAssertInitializer(AssertInitializer node) {
+    InferenceContext.setType(node.condition, typeProvider.boolType);
+    super.visitAssertInitializer(node);
+    return null;
+  }
+
+  @override
   Object visitAssertStatement(AssertStatement node) {
+    InferenceContext.setType(node.condition, typeProvider.boolType);
     super.visitAssertStatement(node);
     _propagateTrueState(node.condition);
     return null;
   }
 
   @override
   Object visitAssignmentExpression(AssignmentExpression node) {
     node.leftHandSide?.accept(this);
     TokenType operator = node.operator.type;
     if (operator == TokenType.EQ ||
@@ skipping 5450 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }