Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/kernel/front_end_adapter.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.
+
+/// Helper classes and methods to adapt between `package:compiler` and
+/// `package:front_end` APIs.
+library compiler.kernel.front_end_adapter;
+
+import 'dart:async';
+
+import 'package:front_end/front_end.dart' as fe;
+
+import '../../compiler_new.dart' as api;
+
+import '../common.dart';
+import '../io/source_file.dart';
+
+/// A front-ends's [FileSystem] that uses dart2js's [api.CompilerInput].
+class CompilerFileSystem implements fe.FileSystem {
+  final api.CompilerInput inputProvider;
+
+  CompilerFileSystem(this.inputProvider);
+
+  @override
+  fe.FileSystemEntity entityForUri(Uri uri) =>
+      new _CompilerFileSystemEntity(uri, this);
+}
+
+class _CompilerFileSystemEntity implements fe.FileSystemEntity {
+  final Uri uri;
+  final CompilerFileSystem fs;
+
+  _CompilerFileSystemEntity(this.uri, this.fs);
+
+  @override
+  Future<String> readAsString() async {
+    api.Input input;
+    try {
+      input = await fs.inputProvider
+          .readFromUri(uri, inputKind: api.InputKind.utf8);
+    } catch (e) {
+      throw new fe.FileSystemException(uri, '$e');
+    }
+    if (input == null) throw new fe.FileSystemException(uri, "File not found");
+    // TODO(sigmund): technically someone could provide dart2js with an input
+    // that is not a SourceFile. Note that this assumption is also done in the
+    // (non-kernel) ScriptLoader.
+    SourceFile file = input as SourceFile;
+    return file.slowText();
+  }
+
+  @override
+  Future<List<int>> readAsBytes() async {
+    api.Input input;
+    try {
+      input = await fs.inputProvider
+          .readFromUri(uri, inputKind: api.InputKind.binary);
+    } catch (e) {
+      throw new fe.FileSystemException(uri, '$e');
+    }
+    if (input == null) throw new fe.FileSystemException(uri, "File not found");
+    return input.data;
+  }
+
+  @override
+  Future<bool> exists() async {
+    try {
+      api.Input input = await fs.inputProvider
+          .readFromUri(uri, inputKind: api.InputKind.binary);
+      return input != null;
+    } catch (e) {
+      return false;
+    }
+  }
+}
+
+/// Report a [message] received from the front-end, using dart2js's
+/// [DiagnosticReporter].
+void reportFrontEndMessage(
+    DiagnosticReporter reporter, fe.CompilationMessage message) {
+  // TODO(sigmund): translate message kinds using message.dart2jsCode
+  MessageKind kind = MessageKind.GENERIC;
+  switch (message.severity) {
+    case fe.Severity.error:
+    case fe.Severity.internalProblem:

[Johnni Winther, 2017/07/28 12:26:42]

I think `internalProblem` should map to a crash (which doesn't have a
corresponding report method on [DiagnosticReporter].

[Siggi Cherem (dart-lang), 2017/07/28 19:15:51]

good point, I guess I can make it throw here, I'll send you a fix shortly.

+      reporter.reportErrorMessage(
+          NO_LOCATION_SPANNABLE, kind, {'text': message.message});
+      break;
+    case fe.Severity.warning:
+      reporter.reportWarningMessage(
+          NO_LOCATION_SPANNABLE, kind, {'text': message.message});
+      break;
+    case fe.Severity.nit:
+      reporter.reportHintMessage(
+          NO_LOCATION_SPANNABLE, kind, {'text': message.message});
+      break;
+    default:
+      throw new UnimplementedError('unhandled severity ${message.severity}');
+  }
+}