Prevent hoisting of certain check nodes, including [HTypeKnown].

tests/compiler/dart2js_extra/17645_test.dart

+// Copyright (c) 2014, 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";
+
+//  Regression test for issue 17645.
+get never => new DateTime.now().millisecondsSinceEpoch == 0;
+
+class A {
+  var foo;
+  A(this.foo);
+}
+
+var log = [];
+
+test1(a, xs) {  // Called with a = [null|exact=A]
+  log.clear();
+  for (var x in xs) {
+    if (a != null) {
+      log.add('${a.foo}.$x');  // a.foo must not be hoisted
+    }
+  }
+  return '$log';
+}
+
+test2(a, xs) {  // Called with a = [exact=A]
+  log.clear();
+  for (var x in xs) {
+    if (a != null) {
+      log.add('${a.foo}.$x');  // a.foo may be hoisted
+    }
+  }
+  return '$log';
+}
+
+test3(a, xs) {  // Called with a = [null|exact=A]
+  log.clear();
+  for (var x in xs) {
+    if (a is A) {
+      log.add('${a.foo}.$x');  // a.foo must not be hoisted
+    }
+  }
+  return '$log';
+}
+
+test4(a, xs) {  // Called with a = [exact=A]
+  log.clear();
+  for (var x in xs) {
+    if (a is A) {
+      log.add('${a.foo}.$x');  // a.foo may be hoisted
+    }
+  }
+  return '$log';
+}
+
+
+main() {
+  var a1 = new A('a1');
+  var a2 = new A('a2');
+
+  Expect.equals('[a1.11]', test1(a1, [11]));
+  Expect.equals('[]', test1(null, [11]));
+
+  Expect.equals('[a1.22]', test2(a1, [22]));
+  Expect.equals('[a2.22]', test2(a2, [22]));
+
+
+  Expect.equals('[a1.33]', test3(a1, [33]));
+  Expect.equals('[]', test3(null, [2]));
+
+  Expect.equals('[a1.44]', test4(a1, [44]));
+  Expect.equals('[a2.44]', test4(a2, [44]));
+}