Specs: fix typos in style2.md, checkin forgotten builtins.md, add guard feature to runloop.md, rena…

sky/specs/utils.md

+Dart Utilities Used By dart:sky
+===============================
+
+The classes defined here are used internally by dart:sky but are
+pretty generic.
+
+```dart
+class Pair<A, B> {
+  const Pair(this.a, this.b);
+  final A a;
+  final B b;
+  int get hashCode => a.hashCode ^ b.hashCode;
+  bool operator==(other) => other is Pair<A, B> && a == other.a && b == other.b;
+}
+
+// MapOfWeakReferences can be implemented in C, using the C Dart API, apparently
+class MapOfWeakReferences<Key, Value> {
+  external operator[](Key key);
+  external operator[]=(Key key, Value value);
+  external bool containsKey(Key key);
+}
+```