Fix dart2js custom hashmap/set not using the custom method for string.

tests/lib/collection/hash_set_test.dart

Index: tests/lib/collection/hash_set_test.dart
diff --git a/tests/lib/collection/hash_set_test.dart b/tests/lib/collection/hash_set_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ab03284ea075001fe9d5ecb85a5297eac2b2f317
--- /dev/null
+++ b/tests/lib/collection/hash_set_test.dart
@@ -0,0 +1,46 @@
+// Copyright (c) 2011, 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:collection";
+import "package:expect/expect.dart";
+
+void main() {
+  // Test customized sets.
+  // Regression test for issue http://dartbug.com/18109
+
+  hash(s) => s.toLowerCase().hashCode;
+  equals(a, b) => a.toLowerCase() == b.toLowerCase();
+
+  for (var m in [
+    new HashSet<String>(equals: equals, hashCode: hash),
+    new LinkedHashSet<String>(equals: equals, hashCode: hash),
+  ]) {
+    m.add("Abel");
+    var prev = "Abel";
+    for (var key in ["Abel", "abel", "ABEL", "Abel"]) {
+      Expect.isTrue(m.contains(key), "contains $key in ${m.runtimeType} $m");
+      Expect.equals(prev, m.lookup(key), "lookup $key in ${m.runtimeType} $m");
+      Expect.isTrue(m.remove(key), "remove $key in ${m.runtimeType} $m");
+      m.add(key);
+      prev = key;
+    }
+  }
+
+  abshash(n) => n.abs();
+  abseq(a, b) => a.abs() == b.abs();
+  for (var m in [
+    new HashSet<int>(equals: abseq, hashCode: abshash),
+    new LinkedHashSet<int>(equals: abseq, hashCode: abshash),
+  ]) {
+    m.add(1);
+    var prev = 1;
+    for (var key in [1, -1, 1]) {
+      Expect.isTrue(m.contains(key), "contains $key in ${m.runtimeType} $m");
+      Expect.equals(prev, m.lookup(key), "lookup $key in ${m.runtimeType} $m");
+      Expect.isTrue(m.remove(key), "remove $key in ${m.runtimeType} $m");
+      m.add(key);
+      prev = key;
+    }
+  }
+}