Add sourceMaps option to $dart2js transformer.

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

Index: sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_in_debug_test.dart b/sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
similarity index 69%
copy from sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_in_debug_test.dart
copy to sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
index 4e5d9e8d467760b025cbca622e6d1aa8fd4a6511..f8ebaec0727ed62368f093772f1a3e47b87b2e77 100644
--- a/sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_in_debug_test.dart
+++ b/sdk/lib/_internal/pub_generated/test/dart2js/includes_source_maps_if_sourceMaps_true_test.dart
@@ -9,17 +9,21 @@ import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("includes source map URLs in a debug build", () {
-    d.dir(
-        appPath,
-        [
-            d.appPubspec(),
-            d.dir(
-                "web",
-                [d.file("main.dart", "void main() => print('hello');")])]).create();
+  integration("includes source maps in a release build if sourceMaps true", () {
+    d.dir(appPath, [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"],
+        args: ["build"],
         output: new RegExp(r'Built \d+ files to "build".'),
         exitCode: 0);