Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/js_emitter/js_emitter.dart

 // Copyright (c) 2013, 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 dart2js.js_emitter;
 
 import 'package:js_ast/src/precedence.dart' as js_precedence;
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 import 'package:js_runtime/shared/embedded_names.dart' show JsBuiltin;
 
@@ skipping 51 matching lines @@
     JavaScriptBackend,
     JavaScriptConstantCompiler,
     Namer,
     RuntimeTypes,
     RuntimeTypesEncoder,
     SetterName,
     Substitution,
     TypeCheck,
     TypeChecks,
     TypeVariableHandler;
-import '../native/native.dart' as native;
 import '../universe/call_structure.dart' show
     CallStructure;
 import '../universe/selector.dart' show
     Selector;
 import '../universe/universe.dart' show
     SelectorConstraints;
 import '../util/util.dart' show
     Setlet;
 
 
 import 'full_emitter/emitter.dart' as full_js_emitter;
 import 'lazy_emitter/emitter.dart' as lazy_js_emitter;
 import 'model.dart';
 import 'program_builder/program_builder.dart';
 import 'startup_emitter/emitter.dart' as startup_js_emitter;
 
 part 'class_stub_generator.dart';
 part 'code_emitter_task.dart';
 part 'helpers.dart';
 part 'interceptor_stub_generator.dart';
 part 'main_call_stub_generator.dart';
 part 'metadata_collector.dart';
 part 'native_emitter.dart';
 part 'native_generator.dart';
 part 'parameter_stub_generator.dart';
 part 'runtime_type_generator.dart';
 part 'type_test_registry.dart';