Add test for checking sources in source map file.

tests/compiler/dart2js/source_map_consistency_helper.dart

Index: tests/compiler/dart2js/source_map_consistency_helper.dart
diff --git a/tests/compiler/dart2js/source_map_consistency_helper.dart b/tests/compiler/dart2js/source_map_consistency_helper.dart
index 3f69eb69bdfd744d8fcbeca57888276471e3d3e8..37a75a985cbf25b33dc652a70cd5b9c3d390a351 100644
--- a/tests/compiler/dart2js/source_map_consistency_helper.dart
+++ b/tests/compiler/dart2js/source_map_consistency_helper.dart
@@ -22,6 +22,10 @@ checkConsistency(Uri outUri) {
   SingleMapping sourceMap = new SingleMapping.fromJson(
       JSON.decode(mapFile.readAsStringSync()));
   Expect.equals(outUri, mapUri.resolve(sourceMap.targetUrl));
+  print('Checking sources');
+  sourceMap.urls.forEach((String url) {
+    Expect.isTrue(new File.fromUri(mapUri.resolve(url)).existsSync());
+  });
 }
 
 String getMapReferenceFromJsOutput(String file) {