Fix main handling in dart2js and add tests.

dart/sdk/lib/_internal/compiler/implementation/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 'dart2jslib.dart' show
     Backend,
     Compiler,
     CompilerTask,
@@ skipping 669 matching lines @@
       }
       for (OutputUnit outputUnit in _constantToOutputUnit.values) {
         allOutputUnits.add(outputUnit);
       }
 
       _assignNamesToOutputUnits(allOutputUnits);
     });
   }
 
   void ensureMetadataResolved(Compiler compiler) {
+    if (compiler.mainApp == null) return;
     _allDeferredImports[_fakeMainImport] = compiler.mainApp;
     var lastDeferred;
     // When detecting duplicate prefixes of deferred libraries there are 4
     // cases of duplicate prefixes:
     // 1.
     // import "lib.dart" deferred as a;
     // import "lib2.dart" deferred as a;
     // 2.
     // import "lib.dart" deferred as a;
     // import "lib2.dart" as a;
@@ skipping 110 matching lines @@
     Element maybePrefix = elements[identifier];
     if (maybePrefix != null && maybePrefix.isPrefix) {
       PrefixElement prefixElement = maybePrefix;
       if (prefixElement.isDeferred) {
         return prefixElement;
       }
     }
     return null;
   }
 }