dart2js: Temporarily store output unit in Fragment.

pkg/compiler/lib/src/js_emitter/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder.dart
index 55786fc4a0d19743028c47b938c44d9ffdab6001..217bcba2323357947635d8db75c6053c89733b91 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder.dart
@@ -122,6 +122,7 @@ class ProgramBuilder {
   MainFragment _buildMainOutput(LibrariesMap librariesMap) {
     // Construct the main output from the libraries and the registered holders.
     MainFragment result = new MainFragment(
+        librariesMap.outputUnit,
         "",  // The empty string is the name for the main output file.
         backend.emitter.staticFunctionAccess(_compiler.mainFunction),
         _buildLibraries(librariesMap),
@@ -136,6 +137,7 @@ class ProgramBuilder {
   DeferredFragment _buildDeferredOutput(MainFragment mainOutput,
                                       LibrariesMap librariesMap) {
     DeferredFragment result = new DeferredFragment(
+        librariesMap.outputUnit,
         backend.deferredPartFileName(librariesMap.name, addExtension: false),
                                      librariesMap.name,
         mainOutput,