Deal better with missing sources in code_transformers and polymer

pkg/polymer/lib/src/build/script_compactor.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.
 
 /// Transfomer that combines multiple dart script tags into a single one.
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 131 matching lines @@
   Future _emitNewEntrypoint(_) {
     if (entryLibraries.isEmpty) {
       // We didn't find code, nothing to do.
       transform.addOutput(transform.primaryInput);
       return null;
     }
 
     return _initResolver()
         .then(_extractUsesOfMirrors)
         .then(_emitFiles)
-        .whenComplete(() => resolver.release());
+        .whenComplete(() {
+          if (resolver != null) resolver.release();
+        });
   }
 
   /// Load a resolver that computes information for every library in
   /// [entryLibraries], then use it to initialize the [recorder] (for import
   /// resolution) and to resolve specific elements (for analyzing the user's
   /// code).
   Future _initResolver() => resolvers.get(transform, entryLibraries).then((r) {
     resolver = r;
     types = new _ResolvedTypes(resolver);
   });
@@ skipping 525 matching lines @@
   for (var c in combinators) {
     if (c is ShowElementCombinator) {
       var show = c.shownNames.toSet();
       elements.retainWhere((e) => show.contains(e.displayName));
     } else if (c is HideElementCombinator) {
       var hide = c.hiddenNames.toSet();
       elements.removeWhere((e) => hide.contains(e.displayName));
     }
   }
 }