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..d1fed3f9786ef82098b9abdefe7eae2b06a2d8e5 100644
--- a/pkg/source_maps/test/parser_test.dart
+++ b/pkg/source_maps/test/parser_test.dart
@@ -45,6 +45,17 @@ main() {
     check(outputExpr, mapping, inputExpr, false);
   });
 
+  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.toMap(), equals(expected));
+    }
+  });
+
   test('parse + json', () {
     var mapping = parse(JSON.encode(EXPECTED_MAP));
     check(outputVar1, mapping, inputVar1, false);