Fix main handling in dart2js and add tests.

dart/sdk/lib/_internal/compiler/implementation/mirrors_used.dart

 // Copyright (c) 2013, 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 dart2js.mirrors_used;
 
 import 'dart2jslib.dart' show
     Compiler,
     CompilerTask,
     Constant,
@@ skipping 83 matching lines @@
   MirrorUsageAnalyzer analyzer;
 
   MirrorUsageAnalyzerTask(Compiler compiler)
       : super(compiler) {
     analyzer = new MirrorUsageAnalyzer(compiler, this);
   }
 
   /// Collect @MirrorsUsed annotations in all libraries.  Called by the
   /// compiler after all libraries are loaded, but before resolution.
   void analyzeUsage(LibraryElement mainApp) {
-    if (compiler.mirrorsLibrary == null) return;
+    if (mainApp == null || compiler.mirrorsLibrary == null) return;
     measure(analyzer.run);
     List<String> symbols = analyzer.mergedMirrorUsage.symbols;
     List<Element> targets = analyzer.mergedMirrorUsage.targets;
     List<Element> metaTargets = analyzer.mergedMirrorUsage.metaTargets;
     compiler.backend.registerMirrorUsage(
         symbols == null ? null : new Set<String>.from(symbols),
         targets == null ? null : new Set<Element>.from(targets),
         metaTargets == null ? null : new Set<Element>.from(metaTargets));
     librariesWithUsage = analyzer.librariesWithUsage;
   }
@@ skipping 467 matching lines @@
       // @MirrorsUsed(targets: fisk)
       //                       ^^^^
       //
       // Instead of saying 'fisk' should pretty print the problematic constant
       // value.
       return spannable;
     }
     return node;
   }
 }