Make invalid @override an warning

compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java

Index: compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java b/compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java
index 4d3a3e1214dc9a67a5f0aa5e4d03052a9f31a1e7..a9dca33279c1c81021f9570f8273f03835cad7ee 100644
--- a/compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java
+++ b/compiler/java/com/google/dart/compiler/resolver/ResolverErrorCode.java
@@ -125,7 +125,7 @@ public enum ResolverErrorCode implements ErrorCode {
   INSTANCE_METHOD_FROM_REDIRECT("Instance methods cannot be referenced from constructor redirects"),
   INSTANCE_METHOD_FROM_STATIC("Instance methods cannot be referenced from static methods"),
   INTERNAL_ERROR("internal error: %s"),
-  INVALID_OVERRIDE_METADATA("Method marked with @override, but does not override any superclass element"),
+  INVALID_OVERRIDE_METADATA(ErrorSeverity.INFO, "Method marked with @override, but does not override any superclass element"),
   INVALID_RETURN_IN_CONSTRUCTOR("Generative constructors cannot return arbitrary expressions"),
   INVALID_TYPE_NAME_IN_CONSTRUCTOR("Invalid type in constructor name"),
   IS_A_CLASS("%s is a class and cannot be used as an expression"),