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

tests/compiler/dart2js/async_compiler_input_provider_test.dart

 // Copyright (c) 2012, 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.
 
 import "dart:async";
 import "dart:io";
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 import 'package:compiler/compiler.dart' as compiler;
-import 'package:compiler/src/filenames.dart';
+import 'package:compiler/implementation/filenames.dart';
 
 const SOURCES = const {
   "/main.dart": """
     import "foo.dart";
     main() => foo();
     """,
   "/foo.dart": """
     library foo;
     import "bar.dart";
     foo() => bar();
@@ skipping 29 matching lines @@
   }));
 }
 
 void handleDiagnostic(Uri uri, int begin, int end, String message,
                       compiler.Diagnostic kind) {
   print(message);
   if (kind != compiler.Diagnostic.VERBOSE_INFO) {
     throw 'Unexpected diagnostic kind $kind';
   }
 }