[dart2dart] Fix assertions for tests to pass in checked mode.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

Index: lib/compiler/implementation/dart_backend/placeholder_collector.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder_collector.dart b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
index 381b5da132b309018184d307446b06854324bca4..6e815c1ab99b07b182e4d9d247a4c2cc2e0339cc 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -394,7 +394,10 @@ class PlaceholderCollector extends Visitor {
       if (send.receiver !== null) tryMakeMemberPlaceholder(send.selector);
     } else if (!element.isErroneous()) {
       if (Elements.isStaticOrTopLevel(element)) {
-        assert(element is VariableElement || element.isAccessor());
+        // TODO(smok): Worth investigating why sometimes we get getter/setter
+        // here and sometimes abstract field.
+        assert(element is VariableElement || element.isAccessor()
+            || element.isAbstractField() || element.isFunction());
         makeElementPlaceholder(send.selector, element);
       } else {
         assert(send.selector is Identifier);