Improve error message and suggest workaround for mixin limitation.

runtime/vm/parser.cc

 // 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.
 
 #include "vm/parser.h"
 
 #include "lib/invocation_mirror.h"
 #include "platform/utils.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
@@ skipping 2524 matching lines @@
   if (current_class().is_mixin_app_alias() ||
       current_class().IsMixinApplication()) {
     // At this point we don't support forwarding constructors
     // that have optional parameters because we don't know the default
     // values of the optional parameters. We would have to compile the super
     // constructor to get the default values. Also, the spec is not clear
     // whether optional parameters are even allowed in this situation.
     // TODO(hausner): Remove this limitation if the language spec indeed
     // allows optional parameters.
     if (func.HasOptionalParameters()) {
+      const Class& super_class = Class::Handle(I, current_class().SuperClass());
       ErrorMsg(ctor_pos,
-               "forwarding constructors must not have optional parameters");
+               "cannot generate an implicit mixin application constructor "
+               "forwarding to a super class constructor with optional "
+               "parameters; add a constructor without optional parameters "
+               "to class '%s' that redirects to the constructor with optional "
+               "parameters and invoke it via super from a constructor of the "
+               "class extending the mixin application",
+               String::Handle(I, super_class.Name()).ToCString());
     }
 
     // Prepare user-defined arguments to be forwarded to super call.
     // The first user-defined argument is at position 2.
     forwarding_args = new ArgumentListNode(Scanner::kNoSourcePos);
     for (int i = 2; i < func.NumParameters(); i++) {
       LocalVariable* param = new LocalVariable(
           Scanner::kNoSourcePos,
           String::ZoneHandle(I, func.ParameterNameAt(i)),
           Type::ZoneHandle(I, Type::DynamicType()));
@@ skipping 8475 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart