Check mixin of black-listed types.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 7d5dcbf30e087af02b5bbb8efdbc1743bac34565..3c3729e54ef56b18d984d313298683d0ce92222b 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -668,6 +668,19 @@ main() { F f = null; }"""]);
   static const MessageKind CANNOT_IMPLEMENT = const MessageKind(
       "Error: '#{type}' cannot be implemented.");
 
+  static const MessageKind CANNOT_MIXIN = const MessageKind(
+      "Error: The type '#{type}' can't be mixed in.",
+      howToFix: "Try removing '#{type}' from the 'with' clause.",
+      examples: const ["""
+class C extends Object with String {}
+
+main() => new C();                       
+""", """
+typedef C = Object with String;
+
+main() => new C();                       
+"""]);
+
   static const MessageKind DUPLICATE_EXTENDS_IMPLEMENTS = const MessageKind(
       "Error: '#{type}' can not be both extended and implemented.");