[ddc] Trust non-nullability of casts

Change-Id: I19f73e7a3e1f65e27a27eb00f6f6d41ffda598af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165141
Reviewed-by: Mark Zhou <markzipan@google.com>

pkg/dev_compiler/lib/src/kernel/nullable_inference.dart

@@ -221,7 +221,10 @@ class NullableInference extends ExpressionVisitor<bool> {
   bool visitIsExpression(IsExpression node) => false;
 
   @override
-  bool visitAsExpression(AsExpression node) => isNullable(node.operand);
+  bool visitAsExpression(AsExpression node) =>
+      _staticallyNonNullable(node.getStaticType(_staticTypeContext))
+          ? false
+          : isNullable(node.operand);
 
   @override
   bool visitSymbolLiteral(SymbolLiteral node) => false;