Implement an LRU cache in dart:_internal and use it for the getter/setter caches in the VM's mirror…

tests/lib/mirrors/lru_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 "dart:mirrors";
+import "package:expect/expect.dart";
+
+newLRUMapWithShift(int shift) {
+  var lib = currentMirrorSystem().libraries[Uri.parse("dart:_internal")];
+  var cls = lib.declarations[#LRUMap];
+  return cls.newInstance(#withShift, [shift]).reflectee;
+}
+
+main() {
+  Expect.throws(() => newLRUMapWithShift(0), (e) => e is Exception);
+
+  for (int shift = 1; shift < 5; shift++) {
+    var map = newLRUMapWithShift(shift);
+    var capacity = (1 << shift) * 3 ~/ 4;
+    for (int value = 0; value < 100; value++) {
+      var key = "$value";
+      map[key] = value;
+      Expect.equals(value, map[key]);
+    }
+    for (int value = 0; value < 100 - capacity - 1; value++) {
+      var key = "$value";
+      Expect.equals(null, map[key]);
+    }
+    for (int value = 100 - capacity; value < 100; value++) {
+      var key = "$value";
+      Expect.equals(value, map[key]);
+    }
+  }
+}