Create and test source mapping for invocations.

pkg/compiler/lib/src/source_file_provider.dart

Index: pkg/compiler/lib/src/source_file_provider.dart
diff --git a/pkg/compiler/lib/src/source_file_provider.dart b/pkg/compiler/lib/src/source_file_provider.dart
index b47fc647a272a54a9b51db50b0584ce9376c7b11..cbc6ff3383cb5be3d3422cfebbf71900054185de 100644
--- a/pkg/compiler/lib/src/source_file_provider.dart
+++ b/pkg/compiler/lib/src/source_file_provider.dart
@@ -103,6 +103,10 @@ abstract class SourceFileProvider {
   Future/*<List<int> | String>*/ call(Uri resourceUri);
 
   relativizeUri(Uri uri) => relativize(cwd, uri, isWindows);
+
+  SourceFile getSourceFile(Uri resourceUri) {
+    return sourceFiles[resourceUri];
+  }
 }
 
 class CompilerSourceFileProvider extends SourceFileProvider {