closure calls (just the invocation part).

frog/leg/resolver.dart

Index: frog/leg/resolver.dart
diff --git a/frog/leg/resolver.dart b/frog/leg/resolver.dart
index 414f2b1e5c218eb8cedbf02b7536e0f70cd47b44..1e8ff66a4aa48d7fa58ccdb6656f2f9d1df8b65f 100644
--- a/frog/leg/resolver.dart
+++ b/frog/leg/resolver.dart
@@ -317,7 +317,15 @@ class FullResolverVisitor extends ResolverVisitor {
     Element receiver = visit(node.receiver);
     visit(node.argumentsNode);
 
-    Identifier selector = node.selector;
+    Identifier selector = node.selector.asIdentifier();
+    if (selector === null) {
+      // We are calling a closure returned from an expression.
+      assert(node.selector.asExpression() !== null);
+      assert(receiver === null);
+      visit(node.selector);
+      return null;
+    }
+
     SourceString name = selector.source;
     // No need to assign an element for a logical operation.
     if (isLogicalOperator(selector)) return null;