Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/async_await_js_transform_test.dart

 // Copyright (c) 2015, 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 "package:expect/expect.dart";
 import "package:compiler/src/js/js.dart";
 import "package:compiler/src/js/rewrite_async.dart";
 import "package:compiler/src/js_backend/js_backend.dart" show StringBackedName;
 
-import "backend_dart/dart_printer_test.dart" show PrintDiagnosticListener;
-
 void testTransform(String source, String expected, AsyncRewriterBase rewriter) {
   Fun fun = js(source);
   Fun rewritten = rewriter.rewrite(fun);
 
   JavaScriptPrintingOptions options = new JavaScriptPrintingOptions();
   SimpleJavaScriptPrintingContext context =
       new SimpleJavaScriptPrintingContext();
   Printer printer = new Printer(options, context);
   printer.visit(rewritten);
   Expect.stringEquals(expected, context.getText());
@@ skipping 1209 matching lines @@
             // return
             return endOfIteration();
           case 2:
             // rethrow
             return uncaughtError(__currentError);
         }
     };
   });
 }""");
 }