Handle is checks (in a naive way: no narrowing for now).

tests/compiler/dart2js/cpa_inference_test.dart

 // Copyright (c) 2012, 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.
 
 import "dart:uri";
 import "../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/types/types.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
@@ skipping 1005 matching lines @@
       } else {
         x = 42;
       }
       x;
     }
     """;
   AnalysisResult result = analyze(source);
   result.checkNodeHasUnknownType('x');
 }
 
+testIsCheck() {
+  final String source = r"""
+    main () {
+      var x = (1 is String);
+      x;
+    }
+    """;
+  AnalysisResult result = analyze(source);
+  result.checkNodeHasType('x', [result.bool]);
+}
+
 void main() {
   testDynamicBackDoor();
   testLiterals();
   testRedefinition();
   testIfThenElse();
   testTernaryIf();
   testWhile();
   testFor1();
   testFor2();
   // testToplevelVariable();  // toplevel variables are not yet supported
@@ skipping 20 matching lines @@
   testSetIndexOperator();
   testInequality();
   // testFieldInitialization(); // TODO(polux)
   testSendWithWrongArity();
   testBigTypesWidening1();
   testBigTypesWidening2();
   testDynamicIsAbsorbing();
   testLists();
   testListWithCapacity();
   testEmptyList();
+  testIsCheck();
 }