Take is checks into account when deriving interceptor classes.

tests/language/interceptor9_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.
+
+// Regression test for dart2js whose codegen in some cases did not take is
+// tests into account when computing the set of classes for interceptors.
+// See http://dartbug.com/17325
+
+import "package:expect/expect.dart";
+import "dart:typed_data";
+
+confuse(x, [y=null]) => new DateTime.now().day == 42 ? y : x;
+
+boom() {
+    var x = confuse(new Uint8List(22), "");
+    Expect.isTrue(x is Uint8List);
+    x.startsWith("a");
+    x.endsWith("u");
+}
+
+main() {
+  try {
+    var f;
+    if (confuse(true)) { // prevent inlining
+      f = boom;
+    }
+    f();
+  } catch (e) {
+    if (e is ExpectException) throw e;
+  }
+}