Incremental compiler detects when library graph changes.

dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart

 // Copyright (c) 2014, 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_incremental;
 
 import 'dart:async' show
     Future;
 
 import 'dart:profiler' show
@@ skipping 137 matching lines @@
   $dart_unsafe_eval.patch(#);
   return main(args);
 }""", updates);
 
 
     jsAst.Printer printer = new jsAst.Printer(_compiler, null);
     printer.blockOutWithoutBraces(mainRunner);
     return printer.outBuffer.getText();
   }
 }
+
+class IncrementalCompilationFailed {
+  final String reason;
+
+  const IncrementalCompilationFailed(this.reason);
+
+  String toString() => "Can't incrementally compile program.\n\n$reason";
+}