Issue 8022. Report compile-time error when using mixing with not Object superclass.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 package com.google.dart.compiler.resolver;
 
 import com.google.dart.compiler.ErrorCode;
 import com.google.dart.compiler.ErrorSeverity;
 import com.google.dart.compiler.SubSystem;
 
 /**
@@ skipping 160 matching lines @@
   NOT_A_CLASS("\"%s\" is not a class"),
   NOT_A_CLASS_OR_INTERFACE("\"%s\" is not a class or interface"),
   // TODO(zundel): error message needs JUnit test (reachable code?)
   NOT_A_LABEL("\"%s\" is not a label"),
   NOT_A_STATIC_FIELD(ErrorSeverity.WARNING, "\"%s\" is not a static field"),
   NOT_A_STATIC_METHOD("\"%s\" is not a static method"),
   NOT_A_TYPE("type \"%s\" expected, but \"%s\" found"),
   // TODO(zundel): error message needs JUnit test (reachable code?)
   NOT_AN_INSTANCE_FIELD("%s is not an instance field"),
   NOT_GENERATIVE_SUPER_CONSTRUCTOR("Constructor '%s' in class '%s' is not generative."),
+  ONLY_OBJECT_MIXIN_SUPERCLASS("Only Object is allowed to be a superclass of a mixin."),
   OPTIONAL_PARAMETERS_CANNOT_START_WITH_UNDER("Optional parameters cannot start with an '_' character"),
   PARAMETER_INIT_OUTSIDE_CONSTRUCTOR("Parameter initializers can only be used in constructors"),
   SUPER_METHOD_INVOCATION_IN_CONSTRUCTOR_INITIALIZER(
       "Super method invocation is not allowed in constructor initializer"),
   PARAMETER_INIT_STATIC_FIELD(
       "Parameter initializer cannot be use to initialize a static field '%s'"),
   // TODO(zundel): error message needs JUnit test (reachable code?)
   PARAMETER_INIT_WITH_REDIR_CONSTRUCTOR(
       "Parameter initializers cannot be used with redirected constructors"),
   // TODO(zundel): error message needs JUnit test
@@ skipping 66 matching lines @@
   @Override
   public SubSystem getSubSystem() {
     return SubSystem.RESOLVER;
   }
 
   @Override
   public boolean needsRecompilation() {
     return true;
   }
 }