Add sourceMaps option to $dart2js transformer.

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

Index: sdk/lib/_internal/pub/test/dart2js/includes_source_maps_if_sourceMaps_true_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/includes_source_maps_if_sourceMaps_true_test.dart
similarity index 76%
copy from sdk/lib/_internal/pub/test/dart2js/includes_source_maps_in_debug_test.dart
copy to sdk/lib/_internal/pub/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
index 69ff7a125d5422d8b56c13623fd3af07ba61fd98..7efba1d1ba66f14cf3fe7fbb0091ca20a4990437 100644
--- a/sdk/lib/_internal/pub/test/dart2js/includes_source_maps_in_debug_test.dart
+++ b/sdk/lib/_internal/pub/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
@@ -9,15 +9,22 @@ import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("includes source map URLs in a debug build", () {
+  integration("includes source maps in a release build if sourceMaps true", () {
     d.dir(appPath, [
-      d.appPubspec(),
+      d.pubspec({
+        'name': 'myapp',
+        'transformers': [{
+          '\$dart2js': {
+            'sourceMaps': true
+          }
+        }]
+      }),
       d.dir("web", [
         d.file("main.dart", "void main() => print('hello');")
       ])
     ]).create();
 
-    schedulePub(args: ["build", "--mode", "debug"],
+    schedulePub(args: ["build"],
         output: new RegExp(r'Built \d+ files to "build".'),
         exitCode: 0);