Handle deferred import without prefix.

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, 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.
 
 library deferred_load;
 
 import 'common/backend_api.dart' show
     Backend;
 import 'common/tasks.dart' show
     CompilerTask;
@@ skipping 891 matching lines @@
 /// A node in the deferred import graph defined by a deferred import directive.
 class _DeclaredDeferredImport implements _DeferredImport {
   final ImportElement declaration;
 
   _DeclaredDeferredImport(this.declaration);
 
   @override
   String computeImportDeferName(Compiler compiler) {
     String result;
     if (declaration.isDeferred) {
-      result = declaration.prefix.name;
+      if (declaration.prefix != null) {
+        result = declaration.prefix.name;
+      } else {
+        // This happens when the deferred import isn't declared with a prefix.
+        assert(compiler.compilationFailed);
+        result = '';
+      }
     } else {
       // Finds the first argument to the [DeferredLibrary] annotation
       List<MetadataAnnotation> metadatas = declaration.metadata;
       assert(metadatas != null);
       for (MetadataAnnotation metadata in metadatas) {
         metadata.ensureResolved(compiler);
         ConstantValue value =
             compiler.constants.getConstantValue(metadata.constant);
         Element element = value.getType(compiler.coreTypes).element;
         if (element == compiler.deferredLibraryClass) {
           ConstructedConstantValue constant = value;
           StringConstantValue s = constant.fields.values.single;
           result = s.primitiveValue.slowToString();
           break;
         }
       }
     }
     assert(result != null);
     return result;
   }
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 }