Associate partial source map with each code block in Universe.generatedCode.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index f547e2d6c6bb0f51086940e2a9524c0067a4d8db..80a242325f3686a0012a8e0d2de36cf1b74f43a0 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -100,7 +100,7 @@ class DartBackend extends Backend {
     // TODO(antonm): Implement this method, if needed.
   }
 
-  String codegen(WorkItem work) {
+  CodeBlock codegen(WorkItem work) {
     // Traverse AST to populate sets of reachable classes and functions.
     log('codegen(${work.element})');
     FunctionExpression function = work.element.parseNode(compiler);