Update DeltaProgram API and documentation.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/incremental_resolved_ast_generator.dart';
 import 'package:front_end/src/base/processed_options.dart';
@@ skipping 44 matching lines @@
     kernelTarget.read(_entryPoint);
 
     // TODO(scheglov) Replace with a better API.
     // Firstly, we don't "write" anything here.
     // Secondly, it catches all the exceptions and write them to `stderr`.
     // This is too interactive and not API-clients friendly.
     await kernelTarget.writeOutline(null);
 
     // TODO(scheglov) Replace with a better API.
     Program program = await kernelTarget.writeProgram(null);
-    return new DeltaProgram({_entryPoint: program});
+    return new DeltaProgram(program);
   }
 
   @override
   void invalidate(String path) => unimplemented();
 
   @override
   void invalidateAll() => unimplemented();
 
   /// Return the [DillTarget] that is used inside of [KernelTarget] to
   /// resynthesize SDK libraries.
@@ skipping 14 matching lines @@
 
 ///// Clears canonical names of [NamedNode] references.
 //class _ClearCanonicalNamesVisitor extends Visitor {
 //  defaultNode(Node node) {
 //    if (node is NamedNode) {
 //      node.reference.canonicalName = null;
 //    }
 //    node.visitChildren(this);
 //  }
 //}