Use JSON.printOn in SourceMapBuilder.

lib/compiler/implementation/source_map_builder.dart

 // Copyright (c) 2012, 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('source_map_builder');
 
 #import('dart:json');
 
 #import('source_file.dart');
 
@@ skipping 47 matching lines @@
 
   String build(List<SourceMappingEntry> mappingEntries, SourceFile targetFile) {
     StringBuffer buffer = new StringBuffer();
     buffer.add('{\n');
     buffer.add('  "version": 3,\n');
     buffer.add('  "mappings": "');
     mappingEntries.forEach((SourceMappingEntry entry) {
       writeEntry(entry, targetFile, buffer);
     });
     buffer.add('",\n');
-    // TODO(podivilov): serialize lists directly to buffer.
-    buffer.add('  "sources": ${JSON.stringify(sourceUrlList)},\n');
-    buffer.add('  "names": ${JSON.stringify(sourceNameList)}\n');
-    buffer.add('}\n');
+    buffer.add('  "sources": ');
+    JSON.printOn(sourceUrlList, buffer);
+    buffer.add(',\n');
+    buffer.add('  "names": ');
+    JSON.printOn(sourceNameList, buffer);
+    buffer.add('\n}\n');
     return buffer.toString();
   }
 
   void writeEntry(SourceMappingEntry entry,
                   SourceFile targetFile,
                   StringBuffer output) {
     if (entry.sourceFile === null) {
       return;
     }
     int targetLine = targetFile.getLine(entry.targetOffset);
@@ skipping 60 matching lines @@
     do {
       int digit = value & VLQ_BASE_MASK;
       value >>= VLQ_BASE_SHIFT;
       if (value > 0) {
         digit |= VLQ_CONTINUATION_BIT;
       }
       output.add(BASE64_DIGITS[digit]);
     } while (value > 0);
   }
 }