Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart

Index: pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
index 0375bc32d8cb514f93a49f78dc9b3a100181d7df..9f2c79f843c012613b8c4c72adef776e67e88954 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
@@ -8,6 +8,8 @@ import '../constants/values.dart' as values;
 import '../dart_types.dart' as types;
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart' as elements;
+import '../resolution/resolution.dart' show
+    TreeElementMapping;
 import '../tree/tree.dart' as tree;
 import '../scanner/scannerlib.dart';
 import '../util/util.dart';
@@ -15,7 +17,7 @@ import 'backend_ast_nodes.dart';
 import 'backend_ast_emitter.dart' show TypeGenerator;
 
 /// Translates the backend AST to Dart frontend AST.
-tree.Node emit(dart2js.TreeElementMapping treeElements,
+tree.Node emit(TreeElementMapping treeElements,
                RootNode root) {
   return new TreePrinter(treeElements).makeDefinition(root);
 }
@@ -28,7 +30,7 @@ bool INSERT_NEW_BACKEND_COMMENT =
 
 /// Converts backend ASTs to frontend ASTs.
 class TreePrinter {
-  dart2js.TreeElementMapping treeElements;
+  TreeElementMapping treeElements;
 
   TreePrinter([this.treeElements]);