Add dump-partial tool.

pkg/front_end/tool/_fasta/dump_partial.dart

Index: pkg/front_end/tool/_fasta/dump_partial.dart
diff --git a/pkg/front_end/tool/_fasta/dump_partial.dart b/pkg/front_end/tool/_fasta/dump_partial.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c800523b01457af9ef9048522de46e91e4f881f8
--- /dev/null
+++ b/pkg/front_end/tool/_fasta/dump_partial.dart
@@ -0,0 +1,72 @@
+// 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.
+
+/// Utility to dump a truncated .dill file generated by the incremental kernel
+/// generator or by the VM's kernel service.
+import 'dart:io';
+import 'package:kernel/kernel.dart';
+import 'package:front_end/src/fasta/kernel/utils.dart';
+
+main(List<String> args) {
+  if (args.length == 0) {
+    print('usage: pkg/front_end/tool/fasta dump_partial '
+        'partial.dill [extra1.dill] ... [extraN.dill]');
+    exitCode = 1;
+  }
+
+  var program = _loadProgram(args);
+  writeProgramToText(program);
+}
+
+/// Creates a program that contains all of the context code marked as external,
+/// and all libraries defined in partial.dill as they are written in that file.
+Program _loadProgram(List<String> args) {
+  List<int> partialInput = new File(args[0]).readAsBytesSync();
+
+  var context = new Program();
+  for (var i = 1; i < args.length; i++) {
+    loadProgramFromBinary(args[i], context);
+  }
+
+  Set<Uri> libraries = _definedLibraries(partialInput, context);
+
+  // By default `package:kernel/binary/ast_from_binary.dart` merges the contents
+  // of libraries that are mentioned in more than one .dill file. In order to
+  // keep the contents of partial.dill intact, we build a new context that
+  // excludes those libraries defined in partial.dill.
+  List<int> contextBytes = serializeProgram(context,
+      filter: (l) => !libraries.contains(l.importUri));
+  var program = new Program();
+  loadProgramFromBytes(contextBytes, program);
+  _updateIsExternal(program, true);
+  loadProgramFromBytes(partialInput, program);
+  return program;
+}
+
+/// Compute the set of libraries defined in [partialDill].
+///
+/// The [context] program contains all other libraries that may be needed to
+/// properly deserialize partialDill.
+///
+/// Note: This function will mutate [context] in place.
+// TODO(sigmund): simplify and get rid of [context]. We could do that with a
+// custom deserialization, but it will be easier to do once .dill has
+// random-access support.
+Set<Uri> _definedLibraries(List<int> partialDill, Program context) {
+  _updateIsExternal(context, true);
+
+  // This implicitly sets `isExternal = false` on all libraries defined in the
+  // partial.dill file.
+  loadProgramFromBytes(partialDill, context);
+  var result = context.libraries
+      .where((l) => !l.isExternal)
+      .map((l) => l.importUri)
+      .toSet();
+  _updateIsExternal(context, false);
+  return result;
+}
+
+void _updateIsExternal(Program program, bool toValue) {
+  program.libraries.forEach((lib) => lib.isExternal = toValue);
+}