Migration: stop using "never" for the result type of an "is" check

Change-Id: I02b80817e46e0544b38dceb7b8a3f92aebede1d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125229
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/nnbd_migration/lib/instrumentation.dart

diff --git a/pkg/nnbd_migration/lib/instrumentation.dart b/pkg/nnbd_migration/lib/instrumentation.dart
index 31e8186..cdf2a95 100644
--- a/pkg/nnbd_migration/lib/instrumentation.dart
+++ b/pkg/nnbd_migration/lib/instrumentation.dart
@@ -142,6 +142,7 @@
   instantiateToBounds,
   isCheckComponentType,
   isCheckMainType,
+  isCheckResultType,
   literal,
   namedParameterNotSupplied,
   nonNullAssertion,

pkg/nnbd_migration/lib/src/edge_builder.dart

diff --git a/pkg/nnbd_migration/lib/src/edge_builder.dart b/pkg/nnbd_migration/lib/src/edge_builder.dart
index 055c273..0bd49be 100644
--- a/pkg/nnbd_migration/lib/src/edge_builder.dart
+++ b/pkg/nnbd_migration/lib/src/edge_builder.dart
@@ -871,7 +871,10 @@
     expression.accept(this);
     _flowAnalysis.isExpression_end(
         node, expression, node.notOperator != null, decoratedType);
-    return DecoratedType(node.staticType, _graph.never);
+    var nullabilityNode = NullabilityNode.forInferredType();
+    _graph.makeNonNullable(
+        nullabilityNode, IsCheckResultTypeOrigin(source, node));
+    return DecoratedType(node.staticType, nullabilityNode);
   }
 
   @override

pkg/nnbd_migration/lib/src/edge_origin.dart

diff --git a/pkg/nnbd_migration/lib/src/edge_origin.dart b/pkg/nnbd_migration/lib/src/edge_origin.dart
index 03cab74..805c7b5 100644
--- a/pkg/nnbd_migration/lib/src/edge_origin.dart
+++ b/pkg/nnbd_migration/lib/src/edge_origin.dart
@@ -251,6 +251,14 @@
   EdgeOriginKind get kind => EdgeOriginKind.isCheckMainType;
 }
 
+/// Edge origin for the nullability of the result of an 'is' check.
+class IsCheckResultTypeOrigin extends EdgeOrigin {
+  IsCheckResultTypeOrigin(Source source, AstNode node) : super(source, node);
+
+  @override
+  EdgeOriginKind get kind => EdgeOriginKind.isCheckResultType;
+}
+
 /// An edge origin used for edges that originated because a literal expression
 /// has a known nullability.
 class LiteralOrigin extends EdgeOrigin {