Add a test for including all libraries from changed to the entry point.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

Index: pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
diff --git a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
index 7761f31bb554c87ac46a83e389ee2928acd535e8..05e88e2a392f24d2618374ace8f9a5bb9c041adf 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -126,6 +126,9 @@ class IncrementalKernelGeneratorImpl implements IncrementalKernelGenerator {
       }
 
       // TODO(scheglov) Add libraries which import changed libraries.
+      // For now the corresponding test works because we use full library
+      // contents to compute signatures (not just API parts). So, every library
+      // that imports a changed one, is affected.
 
       return new DeltaProgram(program);
     });