Issue 1475. Report warning for non-unique Map literal keys.

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

Index: compiler/java/com/google/dart/compiler/resolver/TypeErrorCode.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/TypeErrorCode.java b/compiler/java/com/google/dart/compiler/resolver/TypeErrorCode.java
index f9daf8dfa672e1515f844d89375f41a85ce8f37a..fdbb8ed8a9a712a61786b27776117c2faf28683d 100644
--- a/compiler/java/com/google/dart/compiler/resolver/TypeErrorCode.java
+++ b/compiler/java/com/google/dart/compiler/resolver/TypeErrorCode.java
@@ -35,6 +35,7 @@ public enum TypeErrorCode implements ErrorCode {
   INTERNAL_ERROR("internal error: %s", true),
   IS_STATIC_FIELD_IN("\"%s\" is a static field in \"%s\""),
   IS_STATIC_METHOD_IN("\"%s\" is a static method in \"%s\""),
+  MAP_LITERAL_KEY_UNIQUE("Map literal keys should be unique."),
   MEMBER_IS_A_CONSTRUCTOR("%s is a constructor in %s"),
   MISSING_ARGUMENT("missing argument of type %s"),
   MISSING_RETURN_VALUE("no return value; expected a value of type %s"),