Disallow non-identifiers in for-in.

lib/compiler/implementation/resolver.dart

Index: lib/compiler/implementation/resolver.dart
diff --git a/lib/compiler/implementation/resolver.dart b/lib/compiler/implementation/resolver.dart
index d08b6f60ca5ce0cc308a54015c56b5e351805294..f176afcc3be3e607e0f73b978cf0acdf10730ab1 100644
--- a/lib/compiler/implementation/resolver.dart
+++ b/lib/compiler/implementation/resolver.dart
@@ -2020,7 +2020,8 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
     visitLoopBodyIn(node, node.body, blockScope);
 
     // TODO(lrn): Also allow a single identifier.
-    if ((declaration is !Send || declaration.asSend().selector is !Identifier)
+    if ((declaration is !Send || declaration.asSend().selector is !Identifier
+        || declaration.asSend().receiver != null)
         && (declaration is !VariableDefinitions ||
         !declaration.asVariableDefinitions().definitions.nodes.tail.isEmpty()))
     {