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

tests/compiler/dart2js/sourcemaps/load.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.
 
 /// Loads a source map file and outputs a human-readable version of it.
 
 library load;
 
 import 'dart:convert';
 import 'dart:io';
@@ skipping 56 matching lines @@
       TargetEntry entry = lineEntry.entries[entryIndex];
       int columnStart = entry.column + 1;
       int columnEnd;
       String position;
       if (entryIndex + 1 < lineEntry.entries.length) {
         columnEnd = lineEntry.entries[entryIndex + 1].column + 1;
         position = '$line,$columnStart-$columnEnd';
       } else {
         position = '$line,$columnStart-';
       }
-      if (entry.sourceUrlId != null) {
-        int sourceUrlId = entry.sourceUrlId;
-        int sourceLine = entry.sourceLine + 1;
-        int sourceColumn = entry.sourceColumn + 1;
+      if (entry.sourceUrlId != null || columnEnd == null) {
         if (needsComma) {
           sb.write(',\n');
         }
         sb.write('    {"target": "$position"');
-        sb.write(', "source": "$sourceUrlId:$sourceLine,$sourceColumn"');
-        if (entry.sourceNameId != null) {
-          sb.write(', "name": "${sourceMap.names[entry.sourceNameId]}"');
+        if (entry.sourceUrlId != null) {
+          int sourceUrlId = entry.sourceUrlId;
+          int sourceLine = entry.sourceLine + 1;
+          int sourceColumn = entry.sourceColumn + 1;
+          sb.write(', "source": "$sourceUrlId:$sourceLine,$sourceColumn"');
+          if (entry.sourceNameId != null) {
+            sb.write(', "name": "${sourceMap.names[entry.sourceNameId]}"');
+          }
         }
         sb.write('}');
         needsComma = true;
       }
     }
   }
   sb.write('\n  ]\n');
   sb.write('}');
   return sb.toString();
 }