Fix bad type-inferrence for is-checks in logical and/or expressions.

tests/compiler/dart2js/simple_inferrer_and_or_test.dart

Index: tests/compiler/dart2js/simple_inferrer_and_or_test.dart
diff --git a/tests/compiler/dart2js/simple_inferrer_and_or_test.dart b/tests/compiler/dart2js/simple_inferrer_and_or_test.dart
index 832edd5fd020e4ce178f9bc104f5560780c7350b..a964d41046f891fb67f7cc693f8093ce2fb82e43 100644
--- a/tests/compiler/dart2js/simple_inferrer_and_or_test.dart
+++ b/tests/compiler/dart2js/simple_inferrer_and_or_test.dart
@@ -54,6 +54,15 @@ returnDyn7() {
   return a;
 }
 
+returnDyn8(x) => x;
+
+test8() {
+  var a = "foo";
+  if (a.length == 3) a = 52;
+  if ((false && a is! String) || returnDyn8(a)) return a;
+  return a;
+}
+
 main() {
   returnDyn1();
   returnDyn2();
@@ -62,6 +71,7 @@ main() {
   returnDyn5();
   returnDyn6();
   returnDyn7();
+  test8();
 }
 """;
 
@@ -89,5 +99,6 @@ void main() {
     checkReturn('returnDyn6', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn7', subclassOfInterceptor);
     checkReturn('returnDyn7b', subclassOfInterceptor);
+    checkReturn('returnDyn8', subclassOfInterceptor);
   }));
 }