Rename mixin protos and add a check for undefined mixin name

test/protos/dart_options.proto

 // Experimental options controlling Dart code generation.
 syntax = "proto2";
 
 package dart_options;
 
-// This file must be modified for google internal use,
+// This file must be modified for Google internal use,
 // because custom options only work when the package name
 // agrees with the version of protoc we are using.
 // (The import statement and "google.protobuf." prefix need to be changed.)
 
 import "descriptor_2_5_opensource.proto";
 
-// A mixin to be applied in the 'with' clause of a generated proto in dart.
+// A mixin that can be used in the 'with' clause of the generated Dart class
+// for a proto message.
 message DartMixin {
-  // Class name of the mixin class.
+  // The name of the mixin class.
   optional string name = 1;
 
-  // File from which the mixin class is imported.
+  // A URI pointing to the Dart library that defines the mixin.
   // The generated Dart code will use this in an import statement.
   optional string import_from = 2;
 
-  // Used to apply multiple mixins to a proto.
-  // The mixin listed in parent will always be applied before this one,
-  // making the generated proto implement both mixins.
+  // The name of another mixin to be applied ahead of this one.
+  // The generated class for the message will inherit from all mixins
+  // in the parent chain.
   optional string parent = 3;
 }
 
-// Defines mixins imported from dart files to be used on messages in this file.
-message ImportedMixins {
+// Defines additional Dart imports to be used with messages in this file.
+message Imports {
 
-  // The DartMixins to be imported.
+  // Mixins to be used on messages in this file.
+  // These mixins are in addition to internally defined mixins (e.g PbMapMixin)
+  // and may override them.
+  //
+  // Warning: mixins are experimental. The protoc Dart plugin doesn't check
+  // for name conflicts between mixin class members and generated class members,
+  // so the generated code may contain errors. Therefore, running dartanalyzer
+  // on the generated file is a good idea.
   repeated DartMixin mixins = 1;
 }
 
 extend google.protobuf.FileOptions {
 
-  // Defines the named mixins to be used on messages in this file.
-  // These mixins are in addition to internally defined mixins (e.g PbMapMixin)
-  // and may override them.
-  // Warning: there is no checking at compile time for name conflicts between
-  // identifiers defined in the mixin and proto-generated identifiers.
-  // Running dartanalyzer on the generated file might find some of them.
-  optional ImportedMixins imported_mixins = 28125061;
+  optional Imports imports = 28125061;
 
   // Applies the named mixin to all messages in this file.
   // (May be overridden by the "mixin" option on a message.)
   // For now, "PbMapMixin" is the only available mixin.
   optional string default_mixin = 96128839;
 }
 
 extend google.protobuf.MessageOptions {
 
   // Applies the named mixin.
   // For now, "PbMapMixin" is the only available mixin.
   // The empty string can be used to turn off mixins for this message.
   optional string mixin = 96128839;
 }
 
 extend google.protobuf.FieldOptions {
 
   // Uses the given name for getters, setters and as suffixes for has/clear
   // methods in the generated Dart file. Should be lowerCamelCase.
   optional string dart_name = 28700919;
 }