Use JSON.printOn in SourceMapBuilder.

lib/compiler/implementation/source_map_builder.dart

Index: lib/compiler/implementation/source_map_builder.dart
diff --git a/lib/compiler/implementation/source_map_builder.dart b/lib/compiler/implementation/source_map_builder.dart
index 3e5fb6f991deed84d837ca4f90d1a746b3be4055..ddbe2e67abf29ada04b5932e5487118f0da63408 100644
--- a/lib/compiler/implementation/source_map_builder.dart
+++ b/lib/compiler/implementation/source_map_builder.dart
@@ -65,10 +65,12 @@ class SourceMapBuilder {
       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();
   }