Deal better with missing sources in code_transformers and polymer

pkg/code_transformers/lib/src/resolver_impl.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 code_transformer.src.resolver_impl;
 
 import 'dart:async';
 import 'package:analyzer/analyzer.dart' show parseCompilationUnit;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/constant.dart' show ConstantEvaluator,
@@ skipping 141 matching lines @@
       for (var unreachable in unreachableAssets) {
         changeSet.removedSource(unreachable);
         unreachable.updateContents(null);
         sources.remove(unreachable.assetId);
       }
 
       // Update the analyzer context with the latest sources
       _context.applyChanges(changeSet);
       // Force resolve each entry point (the getter will ensure the library is
       // computed first).
-      _entryLibraries = entryPoints
-          .map((id) => _context.computeLibraryElement(sources[id])).toList();
+      _entryLibraries = entryPoints.map((id) {
+        var source = sources[id];
+        if (source == null) return null;
+        return _context.computeLibraryElement(source);
+      }).toList();
     });
   }
 
   Iterable<LibraryElement> get libraries {
     if (_libraries == null) {
       // Note: we don't use `lib.visibleLibraries` because that excludes the
       // exports seen in the entry libraries.
       _libraries = new Set<LibraryElement>();
       _entryLibraries.forEach(_collectLibraries);
     }
@@ skipping 449 matching lines @@
 
   void apply(ChangeSet changeSet) {
     if (!source.updateContents(content)) return;
     if (source._revision == 1 && source._contents != null) {
       changeSet.addedSource(source);
     } else {
       changeSet.changedSource(source);
     }
   }
 }