Add missing abort on duplicate class fields.

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index ea7c5dde6e0b27d0dad267e3288cfe1791fa80f5..6c10b8dd540f04c8b17e5ff46da57a7b560a098b 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -1030,7 +1030,11 @@ class ClassElement extends ContainerElement
                || element.kind == ElementKind.SETTER) {
       addGetterOrSetter(element, localMembers[element.name], listener);
     } else {
-      localMembers[element.name] = element;
+      Element existing = localMembers.putIfAbsent(element.name, () => element);
+      if (existing !== element) {
+        listener.cancel('duplicate definition', token: element.position());
+        listener.cancel('existing definition', token: existing.position());

[Lasse Reichstein Nielsen, 2012/08/14 10:28:24]

What's the right way to do this?

+      }
     }
   }