Add experimental expando support.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 41c663ca082d9bc6e611831d4aac0799ac2da9c2..69c3823d6015c98351e67cceb814a64ad6364377 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -567,6 +567,22 @@ class Primitives {
     if (value.isNaN()) throw new IllegalArgumentException(str);
     return value;
   }
+
+  static getProperty(object, key) {
+    checkNull(object);
+    if (object is bool || object is num || object is String) {

[sra1, 2012/06/12 07:10:26]

This predicate is in the wrong place.  A public API that uses the API needs to
be doing the check, and possibly exporting the predicate.

+      throw new IllegalArgumentException(object);
+    }
+    return JS('var', '#[#]', object, key);
+  }
+
+  static void setProperty(object, key, value) {
+    checkNull(object);
+    if (object is bool || object is num || object is String) {

[sra1, 2012/06/12 07:10:26]

Ditto.

+      throw new IllegalArgumentException(object);
+    }
+    JS('void', '#[#] = #', object, key, value);
+  }
 }
 
 /**