Revert "Move dart2js from sdk/lib/_internal/compiler to pkg/compiler"

sdk/lib/_internal/pub/lib/src/dart.dart

Index: sdk/lib/_internal/pub/lib/src/dart.dart
diff --git a/sdk/lib/_internal/pub/lib/src/dart.dart b/sdk/lib/_internal/pub/lib/src/dart.dart
index 46d71dd7a5b3c2d57c88e8cc6072d519e89f2a12..a020d9bcf57706a27512702030710e8320ed036a 100644
--- a/sdk/lib/_internal/pub/lib/src/dart.dart
+++ b/sdk/lib/_internal/pub/lib/src/dart.dart
@@ -12,8 +12,8 @@ import 'dart:isolate';
 import 'package:analyzer/analyzer.dart';
 import 'package:path/path.dart' as path;
 
-import '../../../../../../pkg/compiler/lib/compiler.dart' as compiler;
-import '../../../../../../pkg/compiler/lib/src/filenames.dart'
+import '../../../compiler/compiler.dart' as compiler;
+import '../../../compiler/implementation/filenames.dart'
     show appendSlash;
 
 import '../../asset/dart/serialize.dart';