Handle const loop variable.

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 5913cb02da724ee567159b7c654aa8a00b793474..7445a04d254c7344efc8d0ea1fc9ab0a97b2f5b3 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -146,6 +146,7 @@ enum MessageKind {
   CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS,
   CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_CONSTRUCTOR,
   CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_FIELD,
+  CONST_LOOP_VARIABLE,
   CONST_MAP_KEY_OVERRIDES_EQUALS,
   CONST_WITHOUT_INITIALIZER,
   CONSTRUCTOR_CALL_EXPECTED,
@@ -2057,6 +2058,16 @@ void main() {
           examples: const [
               "class C { static final field; } main() => C.field;"]),
 
+      MessageKind.CONST_LOOP_VARIABLE:
+        const MessageTemplate(MessageKind.CONST_LOOP_VARIABLE,
+          "A loop variable cannot be constant.",
+          howToFix: "Try remove the 'const' modifier or "
+                    "replacing it with a 'final' modifier.",
+          examples: const ["""
+void main() {
+  for (const c in []) {}
+}"""]),
+
       MessageKind.MEMBER_USES_CLASS_NAME:
         const MessageTemplate(MessageKind.MEMBER_USES_CLASS_NAME,
           "Member variable can't have the same name as the class it is "