Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/kernel/dart2js_target.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.
 
 library compiler.src.kernel.dart2js_target;
 
 import 'package:kernel/kernel.dart';
 import 'package:kernel/core_types.dart';
 import 'package:kernel/class_hierarchy.dart';
 import 'package:kernel/target/targets.dart';
 
-import 'package:compiler/src/native/native.dart' show maybeEnableNative;
+import '../native/native.dart' show maybeEnableNative;

[Siggi Cherem (dart-lang), 2017/07/21 19:21:31]

this caused problems because sdk/bin/dart2js is importing dart2js.dart in the
lib folder directly without using a package:* URI. I think we should fix that

[Emily Fortuna, 2017/07/21 21:50:51]

add a TODO about it?

[Siggi Cherem (dart-lang), 2017/07/22 00:04:04]

I wanted too, but I wasn't sure where :) - we use relative imports everywhere
because of this.

I filed a bug: https://github.com/dart-lang/sdk/issues/30233

 
 /// A kernel [Target] to configure the Dart Front End for dart2js.
 class Dart2jsTarget extends Target {
   final TargetFlags flags;
 
   Dart2jsTarget(this.flags);
 
   bool get strongMode => flags.strongMode;
 
   String get name => 'dart2js';
@@ skipping 60 matching lines @@
       bool isLocalVariable: false,
       bool isDynamic: false,
       bool isSuper: false,
       bool isStatic: false,
       bool isConstructor: false,
       bool isTopLevel: false}) {
     // TODO(sigmund): implement;
     return new InvalidExpression();
   }
 }