Add more diagnostics to the deferred loading implementation.

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
index 338f22e9c8855e06c9e8427736003122295102f0..a9f53e03811fb66d98390c184750b6a65755000b 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
@@ -1303,7 +1303,12 @@ class FragmentEmitter {
     /// deferred hunk.
     js.Expression initializeLoadedHunkFunction = js.js("""
             function(hash) {
-              initializeDeferredHunk($deferredGlobal[hash]);
+              var hunk = $deferredGlobal[hash];
+              if (hunk == null) {
+                throw "DeferredLoading state error: code with hash '" +
+                    hash + "' was not loaded";
+              }
+              initializeDeferredHunk(hunk);
               #deferredInitialized[hash] = true;
             }""", {
       'deferredInitialized': generateEmbeddedGlobalAccess(DEFERRED_INITIALIZED)