Add sourceMaps option to $dart2js transformer.

sdk/lib/_internal/pub/test/dart2js/omits_source_map_if_sourceMaps_false_test.dart

Index: sdk/lib/_internal/pub/test/dart2js/omits_source_map_if_sourceMaps_false_test.dart
diff --git a/sdk/lib/_internal/pub/test/dart2js/includes_source_maps_in_debug_test.dart b/sdk/lib/_internal/pub/test/dart2js/omits_source_map_if_sourceMaps_false_test.dart
similarity index 67%
copy from sdk/lib/_internal/pub/test/dart2js/includes_source_maps_in_debug_test.dart
copy to sdk/lib/_internal/pub/test/dart2js/omits_source_map_if_sourceMaps_false_test.dart
index 69ff7a125d5422d8b56c13623fd3af07ba61fd98..aaecf9b993b4c605a1679f3be0665533cfd06485 100644
--- a/sdk/lib/_internal/pub/test/dart2js/includes_source_maps_in_debug_test.dart
+++ b/sdk/lib/_internal/pub/test/dart2js/omits_source_map_if_sourceMaps_false_test.dart
@@ -2,16 +2,21 @@
 // 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.
 
-import 'package:scheduled_test/scheduled_test.dart';
-
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("includes source map URLs in a debug build", () {
+  integration("omits source maps from a debug build if sourceMaps false", () {
     d.dir(appPath, [
-      d.appPubspec(),
+      d.pubspec({
+        'name': 'myapp',
+        'transformers': [{
+          '\$dart2js': {
+            'sourceMaps': false
+          }
+        }]
+      }),
       d.dir("web", [
         d.file("main.dart", "void main() => print('hello');")
       ])
@@ -24,9 +29,7 @@ main() {
     d.dir(appPath, [
       d.dir('build', [
         d.dir('web', [
-          d.matcherFile('main.dart.js',
-              contains("# sourceMappingURL=main.dart.js.map")),
-          d.matcherFile('main.dart.js.map', contains('"file": "main.dart.js"'))
+          d.nothing('main.dart.js.map')
         ])
       ])
     ]).validate();