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

tests/lib/collection/hash_map_test.dart

Index: tests/lib/collection/hash_map_test.dart
diff --git a/tests/lib/collection/hash_map_test.dart b/tests/lib/collection/hash_map_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f0f1d541a631d15859e05bb412e1a2e9a8a364b2
--- /dev/null
+++ b/tests/lib/collection/hash_map_test.dart
@@ -0,0 +1,42 @@
+// 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 maps.
+  // 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 HashMap<String,int>(equals: equals, hashCode: hash),
+    new LinkedHashMap<String,int>(equals: equals, hashCode: hash),
+  ]) {
+    m["Abel"] = 42;
+    for (var key in ["Abel", "abel", "ABEL", "Abel"]) {
+      Expect.isTrue(m.containsKey(key), "contains $key in ${m.runtimeType} $m");
+      Expect.equals(42, m[key], "get $key in ${m.runtimeType} $m");
+      Expect.equals(42, m.remove(key), "remove $key in ${m.runtimeType} $m");
+      m[key] = 42;
+    }
+  }
+
+  abshash(n) => n.abs();
+  abseq(a, b) => a.abs() == b.abs();
+  for (var m in [
+    new HashMap<int,int>(equals: abseq, hashCode: abshash),
+    new LinkedHashMap<int,int>(equals: abseq, hashCode: abshash),
+  ]) {
+    m[1] = 42;
+    for (var key in [1, -1, 1]) {
+      Expect.isTrue(m.containsKey(key), "contains $key in ${m.runtimeType} $m");
+      Expect.equals(42, m[key], "get $key in ${m.runtimeType} $m");
+      Expect.equals(42, m.remove(key), "remove $key in ${m.runtimeType} $m");
+      m[key] = 42;
+    }
+  }
+}