Flow analysis: ensure that restrict method doesn't try to promote w…

…rite-captured variables.

Fixes #42066

Change-Id: Ieab80c004b3d331abb81916c3e158416bcd3ce86
Bug: #42066
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153480
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -1804,7 +1804,10 @@ class VariableModel<Variable, Type> {
     bool newUnassigned = unassigned && otherModel.unassigned;
     bool newWriteCaptured = writeCaptured || otherModel.writeCaptured;
     List<Type> newPromotedTypes;
-    if (unsafe) {
+    if (newWriteCaptured) {
+      // Write-captured variables can't be promoted
+      newPromotedTypes = null;
+    } else if (unsafe) {
       // There was an assignment to the variable in the "this" path, so none of
       // the promotions from the "other" path can be used.
       newPromotedTypes = thisPromotedTypes;

pkg/_fe_analyzer_shared/test/flow_analysis/type_promotion/data/bug42066.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2020, 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.
+
+f() {
+  num par2;
+  par2 = 0;
+  if (par2 is! int) return;
+  try {} catch (exception) {
+    throw 'x';
+    () {
+      par2 = 1;
+    };
+  } finally {}
+}