Add warnings and hints to for-in.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index b812fd3347de67b9a77f3d1f3e1edeaa5faba1ca..1f162e6992b181b2bf138d4c50ee5c9dfc431d25 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -197,6 +197,7 @@ enum MessageKind {
   FACTORY_REDIRECTION_IN_NON_FACTORY,
   FINAL_FUNCTION_TYPE_PARAMETER,
   FINAL_WITHOUT_INITIALIZER,
+  FORIN_NOT_ASSIGNABLE,
   FORMAL_DECLARED_CONST,
   FORMAL_DECLARED_STATIC,
   FUNCTION_TYPE_FORMAL_WITH_DEFAULT,
@@ -492,6 +493,11 @@ class MessageTemplate {
         const MessageTemplate(MessageKind.NOT_ASSIGNABLE,
           "'#{fromType}' is not assignable to '#{toType}'."),
 
+      MessageKind.FORIN_NOT_ASSIGNABLE:
+        const MessageTemplate(MessageKind.FORIN_NOT_ASSIGNABLE,
+          "The element type '#{currentType}' of '#{expressionType}' "
+          "is not assignable to '#{elementType}'."),
+
       MessageKind.VOID_EXPRESSION:
         const MessageTemplate(MessageKind.VOID_EXPRESSION,
           "Expression does not yield a value."),