Remove support for mixin typedefs in dart2js.

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 4b6e33adca3c88625adcddd2edc7e0cfcb7ca5eb..a9df3ee6562c9ef83851cf16860e66ec360debba 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -162,7 +162,6 @@ enum MessageKind {
   DEFERRED_LIBRARY_WITHOUT_PREFIX,
   DEFERRED_OLD_SYNTAX,
   DEFERRED_TYPE_ANNOTATION,
-  DEPRECATED_TYPEDEF_MIXIN_SYNTAX,
   DIRECTLY_THROWING_NSM,
   DISALLOWED_LIBRARY_IMPORT,
   DUPLICATE_DEFINITION,
@@ -1633,7 +1632,7 @@ class C extends Object with String {}
 main() => new C();
 """,
             """
-typedef C = Object with String;
+class C = Object with String;
 
 main() => new C();
 """
@@ -2481,19 +2480,6 @@ main() => A.A = 1;
             "main() => +2;  // No longer a valid way to write '2'"
           ]),
 
-      MessageKind.DEPRECATED_TYPEDEF_MIXIN_SYNTAX: const MessageTemplate(
-          MessageKind.DEPRECATED_TYPEDEF_MIXIN_SYNTAX,
-          "'typedef' not allowed here. ",
-          howToFix: "Try replacing 'typedef' with 'class'.",
-          examples: const [
-            """
-class B { }
-class M1 {  }
-typedef C = B with M1;  // Need to replace 'typedef' with 'class'.
-main() { new C(); }
-"""
-          ]),
-
       MessageKind.MIRRORS_EXPECTED_STRING: const MessageTemplate(
           MessageKind.MIRRORS_EXPECTED_STRING,
           "Can't use '#{name}' here because it's an instance of '#{type}' "