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

tests/compiler/dart2js/source_mapping_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 "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import "package:compiler/implementation/dart2jslib.dart";
-import 'package:compiler/implementation/source_file.dart';
+import "package:compiler/src/dart2jslib.dart";
+import 'package:compiler/src/source_file.dart';
 import "mock_compiler.dart";
-import 'package:compiler/implementation/js_backend/js_backend.dart';
+import 'package:compiler/src/js_backend/js_backend.dart';
 
 Future<CodeBuffer> compileAll(SourceFile sourceFile) {
   MockCompiler compiler = new MockCompiler.internal();
   Uri uri = new Uri(path: sourceFile.filename);
   compiler.sourceFiles[uri.toString()] = sourceFile;
   JavaScriptBackend backend = compiler.backend;
   return compiler.runCompiler(uri).then((_) {
     return backend.emitter.oldEmitter.mainBuffer;
   });
 }
@@ skipping 81 matching lines @@
   testSourceMapLocations(FUNCTIONS_TEST);
   testSourceMapLocations(RETURN_TEST);
   testSourceMapLocations(NOT_TEST);
   testSourceMapLocations(UNARY_TEST);
   testSourceMapLocations(BINARY_TEST);
   testSourceMapLocations(SEND_TEST);
   testSourceMapLocations(SEND_SET_TEST);
   testSourceMapLocations(LOOP_TEST);
   testSourceMapLocations(INTERCEPTOR_TEST);
 }