add deprecation message for output=dart

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 147 matching lines @@
 
   setOutputType(String argument) {
     optionsImplyCompilation.add(argument);
     if (argument == '--output-type=dart' ||
         argument == '--output-type=dart-multi') {
       outputLanguage = OUTPUT_LANGUAGE_DART;
       if (!explicitOut) {
         out = currentDirectory.resolve('out.dart');
         sourceMapOut = currentDirectory.resolve('out.dart.map');
       }
+      diagnosticHandler(null, null, null,
+          "--output-type=dart is deprecated. It will remain available "
+          "in Dart 1.11, but will be removed in Dart 1.12.",
+          api.Diagnostic.WARNING);
     }
     passThrough(argument);
   }
 
   String getDepsOutput(Map<Uri, SourceFile> sourceFiles) {
     var filenames = sourceFiles.keys.map((uri) => '$uri').toList();
     filenames.sort();
     return filenames.join("\n");
   }
 
@@ skipping 561 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }