Call resolveLibraryMetadata only when dart:mirrors are loaded

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, 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.compiler_base;
 
 import 'dart:async' show Future;
 
 import '../compiler_new.dart' as api;
 import 'closure.dart' as closureMapping show ClosureTask;
@@ skipping 534 matching lines @@
             resolutionEnqueuer
                 .applyImpact(computeImpactForLibrary(rootLibrary));
           }
           if (librariesToAnalyzeWhenRun != null) {
             for (Uri libraryUri in librariesToAnalyzeWhenRun) {
               resolutionEnqueuer.applyImpact(computeImpactForLibrary(
                   libraryLoader.lookupLibrary(libraryUri)));
             }
           }
         }
-        resolveLibraryMetadata();
+        if (commonElements.mirrorsLibrary != null) {
+          resolveLibraryMetadata();
+        }
         reporter.log('Resolving...');
 
         processQueue(resolutionEnqueuer, mainFunction, libraryLoader.libraries,
             onProgress: showResolutionProgress);
         backend.onResolutionEnd();
         resolutionEnqueuer.logSummary(reporter.log);
 
         _reporter.reportSuppressedMessagesSummary();
 
         if (compilationFailed) {
@@ skipping 124 matching lines @@
         }
       });
     }
     return impactBuilder;
   }
 
   // Resolves metadata on library elements.  This is necessary in order to
   // resolve metadata classes referenced only from metadata on library tags.
   // TODO(ahe): Figure out how to do this lazily.
   void resolveLibraryMetadata() {
-    if (commonElements.mirrorsLibrary == null) return;
+    assert(commonElements.mirrorsLibrary != null);
     for (LibraryElement library in libraryLoader.libraries) {
       if (library.metadata != null) {
         for (MetadataAnnotation metadata in library.metadata) {
           metadata.ensureResolved(resolution);
         }
       }
     }
   }
 
   /**
@@ skipping 966 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }