Moving logic for writing source maps from SourceMapBuilder into the Mapping class.

pkg/source_maps/test/parser_test.dart

Index: pkg/source_maps/test/parser_test.dart
diff --git a/pkg/source_maps/test/parser_test.dart b/pkg/source_maps/test/parser_test.dart
index afdd7fb2c0396a2ebb0c1943cb501ea7fe7a6f27..8f1be3dc5c710ffa67160f25bf6e5ff0c1b8f44e 100644
--- a/pkg/source_maps/test/parser_test.dart
+++ b/pkg/source_maps/test/parser_test.dart
@@ -99,4 +99,15 @@ main() {
     expect(entry.sourceLine, 0);
     expect(entry.sourceNameId, 0);
   });
+
+  test('parse and re-emit', () {
+    for (var expected in [
+        EXPECTED_MAP,
+        MAP_WITH_NO_SOURCE_LOCATION,
+        MAP_WITH_SOURCE_LOCATION,
+        MAP_WITH_SOURCE_LOCATION_AND_NAME]) {
+      var mapping = parseJson(expected);
+      expect(mapping.toJson(), equals(expected));
+    }
+  });
 }