Base JavaScript code position computation on JavaScript tracer.

tests/compiler/dart2js/js_backend_cps_ir_source_information_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.
 
 // Test that the CPS IR code generator generates source information.
 
 library source_information_tests;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 42 matching lines @@
       }
       foundElement = element;
     }
   }
 
   if (foundElement == null) {
     Expect.fail('There is no compiled element called $name');
   }
 
   js.Node ast = compiler.enqueuer.codegen.generatedCode[foundElement];
-  return js.prettyPrint(ast, compiler).getText();
+  return js.prettyPrint(ast, compiler);
 }
 
 runTests(List<TestEntry> tests) {
   for (TestEntry test in tests) {
     Map files = {TEST_MAIN_FILE: test.source};
     asyncTest(() {
       CompilerImpl compiler = compilerFor(
           memorySourceFiles: files, options: <String>['--use-cps-ir']);
       ir.FunctionDefinition irNodeForMain;
 
@@ skipping 69 matching lines @@
   print('Hello');
   print('World');
 }
 """, const ['memory:test.dart:[2,3]',
             'memory:test.dart:[3,3]']),
 ];
 
 void main() {
   runTests(tests);
 }