Update SOURCE_KIND when a missing source file appears

pkg/analyzer/lib/src/context/source.dart

 // Copyright (c) 2015, 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 analyzer.src.context.source;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 241 matching lines @@
               'package:$name/${sourceUri.path.substring(uri.path.length)}');
         }
       }
     });
     return packageUri;
   }
 
   /**
    * Return a source object representing the URI that results from resolving
    * the given (possibly relative) contained URI against the URI associated
-   * with an existing source object, or `null` if the URI could not be resolved.
+   * with an existing source object.
+   *
+   * Returns null if no resolver can handle the URI. If the source file is

[Brian Wilkerson, 2016/02/26 15:07:32]

nit: "Returns null" --> "Return `null`"

[skybrian, 2016/02/26 18:09:01]

Okay, but FYI this is the opposite of normal dartdoc style:
https://www.dartlang.org/effective-dart/documentation/#prefer-starting-functi...

[Brian Wilkerson, 2016/02/26 18:35:57]

I'm aware, but it's consistent with most of the rest of the dartdocs in analyzer
and analysis server.

+   * missing, returns a non-null Source that will have a negative

[Brian Wilkerson, 2016/02/26 15:07:32]

nit: "returns" --> "return"

+   * modificationStamp.
    *
    * @param containingSource the source containing the given URI
    * @param containedUri the (possibly relative) URI to be resolved against the
    *        containing source
    * @return the source representing the contained URI
    * @throws AnalysisException if either the contained URI is invalid or if it
    *         cannot be resolved against the source object's URI
    */
   Source _internalResolveUri(Source containingSource, Uri containedUri) {
     if (!containedUri.isAbsolute) {
@@ skipping 29 matching lines @@
     for (UriResolver resolver in resolvers) {
       Source result = resolver.resolveAbsolute(containedUri, actualUri);
       if (result != null) {
         return result;
       }
     }
 
     return null;
   }
 }