Add more diagnostics to the deferred loading implementation.

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.js_emitter.startup_emitter.model_emitter;
 
 /// The name of the property that stores the tear-off getter on a static
 /// function.
 ///
 /// This property is only used when isolates are used.
@@ skipping 1285 matching lines @@
         "function(hash) { return !!#deferredInitialized[hash]; }", {
       'deferredInitialized': generateEmbeddedGlobalAccess(DEFERRED_INITIALIZED)
     });
     globals.add(new js.Property(
         js.string(IS_HUNK_INITIALIZED), isHunkInitializedFunction));
 
     /// See [emitEmbeddedGlobalsForDeferredLoading] for the format of the
     /// 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)
     });
 
     globals.add(new js.Property(
         js.string(INITIALIZE_LOADED_HUNK), initializeLoadedHunkFunction));
 
     return globals;
   }
@@ skipping 227 matching lines @@
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(
         js.js.statement("setOrUpdateLeafTags(#);", js.objectLiteral(leafTags)));
     statements.addAll(subclassAssignments);
 
     return wrapPhase('nativeSupport', statements);
   }
 }