Specs: turns out my Expando-based WeakMap thing won't work :-(

sky/specs/style2.md

 Sky Style Language
 ==================
 
 Note: This is a work in progress that will eventually replace
 (style.md)[style.md].
 
 The Sky style API looks like the following:
 
 ```dart
 
@@ skipping 40 matching lines @@
   var style2 = new StyleDeclaration.clone(style);
 ```
 
 The dart:sky library contains the following to define this API:
 
 ```dart
 import 'dart:mirrors';
 import 'dart:math';
 
 class WeakMap<Key, Value> {
-  Expando<Value> _map = new Expando<Value>();
+  // This is not actually a weak map right now, because Dart doesn't let us have weak references.
+  // We should fix that, or else we're going to keep alive every object you ever tear off through
+  // the StyleDeclaration API, even if you never use it again, until the StyleDeclaration object
+  // itself is GC'ed, which is likely when the element is GC'ed, which is likely never.
+  Map<Key, Value> _map = new Map<Key, Value>();
   operator[](Key key) => _map[key];
   operator[]=(Key key, Value value) => _map[key] = value;
-  bool containsKey(Key key) => _map[key] != null;
+  bool containsKey(Key key) => _map.containsKey(key);
 }
 
 typedef void StringSetter(Symbol propertySymbol, StyleDeclaration declaration, String value);
 typedef String StringGetter(Symbol propertySymbol, StyleDeclaration declaration);
 typedef Property ObjectConstructor(Symbol propertySymbol, StyleDeclaration declaration);
 
 class PropertyTable {
   const PropertyTable({this.symbol, this.inherited, this.stringGetter, this.stringSetter, this.objectConstructor});
   final Symbol symbol;
   final bool inherited;
@@ skipping 267 matching lines @@
   registerProperty(new PropertyTable(   
     symbol: #transform,
     inherited: false,
     stringSetter: transformPropertyStringSetter,
     stringGetter: transformPropertyStringGetter,
     objectConstructor: (Symbol propertySymbol, StyleDeclaration declaration) =>
                          new TransformProperty(propertySymbol, declaration)));
 }
 ```