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

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

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index c3cb4e25f823bcf8b8dfcb53c19b12af1d474b0c..241d53009526d08ffb3173aa8b6ea8e4443a04f1 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -5195,8 +5195,16 @@ class ResolverVisitor extends ScopedVisitor {
     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;