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

tests/compiler/dart2js/backend_dart/end2end_test.dart

 // Copyright (c) 2014, 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.
 
 /// End-to-end test of the dart2dart compiler.
 library dart_backend.end2end_test;
 
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/src/dart2jslib.dart';
-import 'package:compiler/src/dart_backend/dart_backend.dart';
+import 'package:compiler/implementation/dart2jslib.dart';
+import 'package:compiler/implementation/dart_backend/dart_backend.dart';
 import 'package:expect/expect.dart';
 
 import '../../../../pkg/analyzer2dart/test/test_helper.dart' hide TestSpec;
 import '../../../../pkg/analyzer2dart/test/end2end_data.dart';
 
 import 'test_helper.dart';
 
 main() {
   performTests(TEST_DATA, asyncTester, (TestSpec result) {
     asyncTest(() => compilerFor(result.input).then((Compiler compiler) {
       String expectedOutput = result.output.trim();
       compiler.phase = Compiler.PHASE_COMPILING;
       DartBackend backend = compiler.backend;
       backend.assembleProgram();
       String output = compiler.assembledCode.trim();
       Expect.equals(expectedOutput, output,
           '\nInput:\n${result.input}\n'
           'Expected:\n$expectedOutput\n'
           'Actual:\n$output\n');
     }));
   });
 }