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

tests/compiler/dart2js/backend_dart/sexpr_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.
 
 library dart_backend.sexpr_test;
 
 import 'dart:async';
 
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/implementation/dart2jslib.dart';
-import 'package:compiler/implementation/cps_ir/cps_ir_nodes.dart';
-import 'package:compiler/implementation/cps_ir/cps_ir_nodes_sexpr.dart';
+import 'package:compiler/src/dart2jslib.dart';
+import 'package:compiler/src/cps_ir/cps_ir_nodes.dart';
+import 'package:compiler/src/cps_ir/cps_ir_nodes_sexpr.dart';
 import 'package:expect/expect.dart';
 
 import '../compiler_helper.dart' hide compilerFor;
 import 'sexpr_unstringifier.dart';
 import 'test_helper.dart';
 
 const String CODE = """
 class Foo {
   static int x = 1;
   foo() => 42;
@@ skipping 142 matching lines @@
           ];
 
   asyncTest(() => testStringifier(CODE, tokens).then((List<String> sexprs) {
     final functions = testUnstringifier(sexprs);
 
     // Ensure that
     // stringified(CODE) == stringified(unstringified(stringified(CODE)))
     Expect.listEquals(sexprs, stringifyAll(functions));
   }));
 }