Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/ssa/ssa.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.
 
 library ssa;
 
 import 'dart:collection';
 
+import 'package:js_runtime/shared/embedded_names.dart';
+
 import '../closure.dart';
 import '../constant_system_dart.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart2jslib.dart' hide Selector, TypedSelector;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../elements/modelx.dart'
     show ElementX,
          VariableElementX,
          ConstructorBodyElementX;
-
 import '../io/source_information.dart';
 import '../js/js.dart' as js;
 import '../js_backend/js_backend.dart';
-import '../js_emitter/js_emitter.dart' show CodeEmitterTask, NativeEmitter;
+import '../js_emitter/js_emitter.dart' show
+    CodeEmitterTask,
+    NativeEmitter;
+import '../messages.dart';
 import '../native/native.dart' as native;
 import '../resolution/operators.dart';
+import '../resolution/resolution.dart' show
+    TreeElements;
 import '../resolution/semantic_visitor.dart';
 import '../tree/tree.dart' as ast;
 import '../types/types.dart';
-import '../types/constants.dart' show computeTypeMask;
+import '../types/constants.dart' show
+    computeTypeMask;
 import '../universe/universe.dart';
 import '../util/util.dart';
-import '../js/rewrite_async.dart';
-import 'package:js_runtime/shared/embedded_names.dart';
+import '../world.dart' show
+    ClassWorld,
+    World;
 
 part 'builder.dart';
 part 'codegen.dart';
 part 'codegen_helpers.dart';
 part 'interceptor_simplifier.dart';
 part 'invoke_dynamic_specializers.dart';
 part 'nodes.dart';
 part 'optimize.dart';
 part 'types.dart';
 part 'types_propagation.dart';
 part 'validate.dart';
 part 'variable_allocator.dart';
 part 'value_range_analyzer.dart';
 part 'value_set.dart';