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

tests/compiler/dart2js/simple_inferrer_and_or_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'type_mask_test_helper.dart';
 
 const String TEST = """
@@ skipping 36 matching lines @@
 
 returnDyn7b(x) => x;
 
 returnDyn7() {
   var a = "foo";
   if (a.length == 3) a = 52;
   if ((a is int) || (a is String && true)) returnDyn7b(a);
   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();
   returnDyn3();
   returnDyn4();
   returnDyn5();
   returnDyn6();
   returnDyn7();
+  test8();
 }
 """;
 
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkReturn(String name, type) {
       var element = findElement(compiler, name);
       Expect.equals(type,
           simplify(typesInferrer.getReturnTypeOfElement(element), compiler));
     }
 
     var subclassOfInterceptor =
         findTypeMask(compiler, 'Interceptor', 'nonNullSubclass');
 
     checkReturn('returnDyn1', subclassOfInterceptor);
     checkReturn('returnDyn2', subclassOfInterceptor);
     checkReturn('returnDyn3', subclassOfInterceptor);
     checkReturn('returnDyn4', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn5', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn6', compiler.typesTask.dynamicType.nonNullable());
     checkReturn('returnDyn7', subclassOfInterceptor);
     checkReturn('returnDyn7b', subclassOfInterceptor);
+    checkReturn('returnDyn8', subclassOfInterceptor);
   }));
 }