Implement the generate-kernel api by invoking dartk. Add example that uses this API to generate a by

pkg/front_end/lib/kernel_generator.dart

Index: pkg/front_end/lib/kernel_generator.dart
diff --git a/pkg/front_end/lib/kernel_generator.dart b/pkg/front_end/lib/kernel_generator.dart
index 3b3508655d2395d5ffd1464a6dbce58792773ed2..3107e7f92ee2cc117574b01c4ea450d9a991c7d5 100644
--- a/pkg/front_end/lib/kernel_generator.dart
+++ b/pkg/front_end/lib/kernel_generator.dart
@@ -5,9 +5,13 @@
 /// Defines the front-end API for converting source code to Dart Kernel objects.
 library front_end.kernel_generator;
 
-import 'dart:async';
+import 'compilation_error.dart';
 import 'compiler_options.dart';
-import 'package:kernel/kernel.dart' as kernel;
+import 'dart:async';
+
+// TODO(sigmund): move loader logic under front_end/lib/src/kernel/
+import 'package:kernel/analyzer/loader.dart';
+import 'package:kernel/kernel.dart';
 
 /// Generates a kernel representation of the program whose main library is in
 /// the given [source].
@@ -24,8 +28,12 @@ import 'package:kernel/kernel.dart' as kernel;
 ///
 /// TODO(paulberry): will the VM have a pickled version of the SDK inside it? If
 /// so, then maybe this method should not convert SDK libraries to kernel.
-Future<kernel.Program> kernelForProgram(Uri source, CompilerOptions options) =>
-    throw new UnimplementedError();
+Future<Program> kernelForProgram(Uri source, CompilerOptions options) async {
+  var loader = await _createLoader(options);
+  Program program = loader.loadProgram(source);
+  _reportErrors(loader.errors, options.onError);
+  return program;
+}
 
 /// Generates a kernel representation of the build unit whose source files are
 /// in [sources].
@@ -46,13 +54,50 @@ Future<kernel.Program> kernelForProgram(Uri source, CompilerOptions options) =>
 /// are also listed in [sources], otherwise an error results.  (It is not
 /// permitted to refer to a part file declared in another build unit).
 ///
-/// The return value is a [kernel.Program] object with no main method set.
+/// The return value is a [Program] object with no main method set.
 /// TODO(paulberry): would it be better to define a data type in kernel to
 /// represent a bundle of all the libraries in a given build unit?
 ///
 /// TODO(paulberry): does additional information need to be output to allow the
 /// caller to match up referenced elements to the summary files they were
 /// obtained from?
-Future<kernel.Program> kernelForBuildUnit(
-        List<Uri> sources, CompilerOptions options) =>
-    throw new UnimplementedError();
+Future<Program> kernelForBuildUnit(
+        List<Uri> sources, CompilerOptions options) async {
+  var repository = new Repository();
+  var loader = await _createLoader(options, repository: repository);
+  sources.forEach(loader.loadLibrary);

[Paul Berry, 2016/12/04 09:09:52]

Does this work correctly if one of the sources is a part file?

[Siggi Cherem (dart-lang), 2016/12/05 21:00:50]

Good point - I'll add a TODO, it doesn't crash but I don't believe it is doing
the right thing either.

+  Program program = new Program(repository.libraries);
+  _reportErrors(loader.errors, options.onError);
+  return program;
+}
+
+Future<DartLoader> _createLoader(CompilerOptions options,
+    {Repository repository}) async {
+  var kernelOptions = _convertOptions(options);
+  var packages = await createPackages(options.packagesFilePath);
+  return new DartLoader(
+      repository ?? new Repository(), kernelOptions, packages);
+}
+
+DartOptions _convertOptions(CompilerOptions options) {

[Paul Berry, 2016/12/04 09:09:52]

How feasible would it be to get dartk to respect CompilerOptions.fileSystem?  It
would make unit testing easier, since we wouldn't have to have all of our unit
tests create temporary directories.

[Siggi Cherem (dart-lang), 2016/12/05 21:00:50]

I believe it shouldn't take too much work to do so. Most IO is done via the
SourceFactory today. The only direct access to IO is for finding the .packages
file, but that should be easy to change.

+  return new DartOptions(
+      sdk: options.sdkPath,
+      packagePath: options.packagesFilePath,
+      declaredVariables: options.declaredVariables);
+}
+
+void _reportErrors(List errors, ErrorHandler onError) {
+  if (onError == null) return;
+  for (var error in errors) {
+    onError(new _DartkError(error));
+  }
+}
+
+// TODO(sigmund): delete this class. Dartk should not format errors itself, we
+// should just pass them along.
+class _DartkError implements CompilationError {
+  String get correction => null;
+  String get span => null;
+  final String message;
+  _DartkError(this.message);
+}