add deprecation message for output=dart

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index a7d02be97507925264ae24b94f306c4ae93c95bc..c3547fa06536d7ee34336d97d7eac55a12f7db2d 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -165,6 +165,10 @@ Future<api.CompilationResult> compile(List<String> argv) {
         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);
   }