Incremental compiler detects when library graph changes.

dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart

Index: dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart
diff --git a/dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart b/dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart
index 5ece461735569f7a718fde9bcb017bfbf64ab5d0..b78716d4a03be92e09f7e69f2ca62d1a98960085 100644
--- a/dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart
+++ b/dart/pkg/dart2js_incremental/lib/dart2js_incremental.dart
@@ -155,3 +155,11 @@ function dartMainRunner(main, args) {
     return printer.outBuffer.getText();
   }
 }
+
+class IncrementalCompilationFailed {
+  final String reason;
+
+  const IncrementalCompilationFailed(this.reason);
+
+  String toString() => "Can't incrementally compile program.\n\n$reason";
+}