Revert "remove serialization. it's moved to github"

pkg/serialization/lib/src/serialization_helpers.dart

+// Copyright (c) 2012, 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.
+
+/**
+ * This contains extra functions and classes useful for implementing
+ * serialiation. Some or all of these will be removed once the functionality is
+ * available in the core library.
+ */
+library serialization_helpers;
+
+import 'dart:collection';
+
+/**
+ * A named function of one argument that just returns it. Useful for using
+ * as a default value for a function parameter or other places where you want
+ * to concisely provide a function that just returns its argument.
+ */
+doNothing(x) => x;
+
+/** Concatenate two lists. Handle the case where one or both might be null. */
+// TODO(alanknight): Remove once issue 5342 is resolved.
+Iterable append(Iterable a, Iterable b) {
+  if (a == null) {
+    return (b == null) ? [] : new List.from(b);
+  }
+  if (b == null) return new List.from(a);
+  var result = new List.from(a);
+  result.addAll(b);
+  return result;
+}
+
+/** Helper function for PrimitiveRule to tell which objects it applies to. */
+bool isPrimitive(object) {
+  return identical(object, null) || object is num || object is String ||
+      identical(object, true) || identical(object, false);
+}
+
+/**
+ * Given either an Iterable or a Map, return a map. For a Map just return it.
+ * For an iterable, return a Map from the index to the value at that index.
+ *
+ * Used to iterate polymorphically between List-like and Map-like things.
+ * For example, keysAndValues(["a", "b", "c"]).forEach((key, value) => ...);
+ * will loop over the key/value pairs 1/"a", 2/"b", 3/"c", as if the argument
+ * was a Map from integer keys to string values.
+ */
+Map keysAndValues(x) {
+  if (x is Map) return x;
+  if (x is List) return x.asMap();
+  if (x is Iterable) return x.toList().asMap();
+  throw new ArgumentError("Invalid argument, expected Map or Iterable, got $x");
+}
+
+/**
+ * Lets you iterate polymorphically between
+ * List-like and Map-like things, but making them behave like Lists, instead
+ * of behaving like Maps.
+ * So values(["a", "b", "c"]).forEach((value) => ...);
+ * will loop over the values "a", "b", "c", as if it were a List of values.
+ * Only supports forEach() and map() operations because that was all I needed
+ * for the moment.
+ */
+Iterable values(x) {
+  if (x is Iterable) return x;
+  if (x is Map) return x.values;
+  throw new ArgumentError("Invalid argument, expected Map or Iterable, got $x");
+}
+
+/**
+ * Iterate over [collection] and return a new collection of the same type
+ * where each value has been transformed by [f]. For iterables and sets, this
+ * is equivalent to [map]. For a Map, it returns a new Map with the same keys
+ * and the corresponding values transformed by [f].
+ */
+mapValues(collection, Function f) {
+  if (collection is Set) return collection.map(f).toSet();
+  if (collection is Iterable) return collection.map(f).toList();
+  if (collection is Map) return new Map.fromIterables(collection.keys,
+      collection.values.map(f));
+  throw new ArgumentError("Invalid argument, expected Map or Iterable, "
+      "got $collection");
+}
+
+/**
+ * This acts as a stand-in for some value that cannot be hashed. We can't
+ * just use const Object() because the compiler will fold them together.
+ */
+class _Sentinel {
+  final _wrappedObject;
+  const _Sentinel(this._wrappedObject);
+}