Fix for issue 8781: it is not ok to move a HTypeGuard, so mark it as "unpure".

tests/language/field_increment_bailout_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.
+
+// dart2js regression test for issue 8781.
+
+class N {
+  var outgoing;
+  var incoming;
+  N(this.outgoing, this.incoming);
+}
+
+class A {
+  int offset = 0;
+  var list;
+  var node;
+
+  A(node) : node = node, list = node.outgoing;
+
+  next() {
+    // dart2js used to update [offset] twice: once in the optimized
+    // version, which would bailout to the non-optimized version
+    // because [list] is not an Array, and once in the non-optimized
+    // version.
+    var edge = list[offset++];
+    if (list == node.outgoing) {
+      list = node.incoming;
+      offset = 0;
+    } else
+      list = null;
+    return edge;
+  }
+}
+
+class L {
+  final list;
+  L(this.list);
+  // Use noSuchMethod to defeat type inferencing.
+  noSuchMethod(mirror) => mirror.invokeOn(list);
+}
+
+main () {
+  var o = new A(new N(new L([1]), new L([2])));
+
+  for (var i = 1; i <= 2; i++)
+    Expect.equals(i, o.next());
+
+  Expect.equals(null, o.list);
+}