Allow for-in to use a previously declared identifier as variable.

frog/leg/resolver.dart

Index: frog/leg/resolver.dart
diff --git a/frog/leg/resolver.dart b/frog/leg/resolver.dart
index 6a8342687b8c928376d1e8912624278d79582889..38e3e0cfe6e62a6365635d93d25f40392d291e42 100644
--- a/frog/leg/resolver.dart
+++ b/frog/leg/resolver.dart
@@ -521,12 +521,12 @@ class FullResolverVisitor extends ResolverVisitor {
     visitIn(declaration, scope);
     visitIn(node.body, scope);
     // TODO(lrn): Also allow a single identifier.
-    if (declaration is !VariableDefinitions ||
-        !declaration.definitions.nodes.tail.isEmpty()) {
-      // The variable declaration is either not a declaration, or it's
-      // declaring more than one variable.
-      error(node.declaredIdentifier, MessageKind.GENERIC,
-            ["Invalid variable declaration in for-in"]);
+    if ((declaration is !Send || declaration.selector is !Identifier) &&
+        (declaration is !VariableDefinitions ||
+         !declaration.definitions.nodes.tail.isEmpty())) {
+      // The variable declaration is either not an identifier, not a
+      // declaration, or it's declaring more than one variable.
+      error(node.declaredIdentifier, MessageKind.INVALID_FOR_IN, []);
     }
   }