dart2js: allow empty main fragment in emitter.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index bd179fa35313ddae4e5e2bae74757dcd1bd61419..c7acbcae9d8a583936e9e8b1e2489d9745ac11ff 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -1270,6 +1270,8 @@ class OldEmitter implements Emitter {
     emitMangledNames(mainOutput);
 
     Map<Element, ClassBuilder> descriptors = elementDescriptors[mainFragment];
+    if (descriptors == null) descriptors = const {};
+
     checkEverythingEmitted(descriptors.keys);
 
     CodeBuffer libraryBuffer = new CodeBuffer();