Update SOURCE_KIND when a missing source file appears

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

Index: pkg/analyzer/lib/src/context/source.dart
diff --git a/pkg/analyzer/lib/src/context/source.dart b/pkg/analyzer/lib/src/context/source.dart
index faf8eaab12890fa547a34665afbe3b9d69ee44ea..659b2ab90858a663087e0a50d4e8a673b69d8677 100644
--- a/pkg/analyzer/lib/src/context/source.dart
+++ b/pkg/analyzer/lib/src/context/source.dart
@@ -259,7 +259,11 @@ class SourceFactoryImpl implements SourceFactory {
   /**
    * 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