Add no-info mappings at start of out.js and after mapped functions

tests/compiler/dart2js/sourcemaps/js_tracer.dart

 // Copyright (c) 2016, 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 sourcemap.js_tracer;
 
 import 'package:compiler/src/io/source_information.dart';
 import 'package:compiler/src/io/position_information.dart';
 import 'package:compiler/src/js/js.dart' as js;
 import 'sourcemap_helper.dart';
@@ skipping 64 matching lines @@
         text = ['while(', whileNode.condition, ') ...'];
         break;
       case StepKind.DO_CONDITION:
         js.Do doNode = node;
         text = ['do {... } (', doNode.condition, ')'];
         break;
       case StepKind.SWITCH_EXPRESSION:
         js.Switch switchNode = node;
         text = ['switch(', switchNode.key, ') ...'];
         break;
+      case StepKind.NO_INFO:
+        break;
     }
     createTraceStep(kind, node,
         offset: offset,
         sourceLocation:
             getSourceLocation(sourceInformation, sourcePositionKind),
         text: text);
   }
 
   void createTraceStep(StepKind kind, js.Node node,
       {Offset offset, List text, String note, SourceLocation sourceLocation}) {
@@ skipping 29 matching lines @@
         branch = '$value';
         break;
     }
     graph.pushBranch(branch);
   }
 
   void popBranch() {
     graph.popBranch();
   }
 }