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

tests/compiler/dart2js/compiler_alt.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.
 
 /// Alternative compile method for dart2js commandline that stores the compiler
 /// for later inspection.
 ///
 /// Use this in testing to inspect the compiler after compilation by setting
-/// the `compileFunc` variable in `package:compiler/src/dart2js.dart`
+/// the `compileFunc` variable in `package:compiler/implementation/dart2js.dart`
 /// to [compiler] before calling the `internalMain` function.
 
 library dart2js.alt;
 
 import 'dart:async';
 import 'package:compiler/compiler.dart';
-import 'package:compiler/src/apiimpl.dart';
+import 'package:compiler/implementation/apiimpl.dart';
 
 Compiler compiler;
 
 Future<String> compile(Uri script,
                        Uri libraryRoot,
                        Uri packageRoot,
                        CompilerInputProvider inputProvider,
                        DiagnosticHandler handler,
                        [List<String> options = const [],
                         CompilerOutputProvider outputProvider,
@@ skipping 12 matching lines @@
                           options,
                           environment);
   return compiler.run(script).then((_) {
     String code = compiler.assembledCode;
     if (code != null && outputProvider != null) {
       code = ''; // Non-null signals success.
     }
     return code;
   });
 }