Handle 'part of' declarations with URI correctly in Fasta.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index 15a24a74dbcfbb88fdd7f5bd612c226187aa11cd..2cabd7831518da345cd09b7e59a99851a8ffe50c 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -576,11 +576,12 @@ class OutlineBuilder extends UnhandledListener {
   }
 
   @override
-  void endPartOf(Token partKeyword, Token semicolon) {
+  void endPartOf(Token partKeyword, Token semicolon, bool hasName) {
     debugEvent("endPartOf");
-    String name = pop();
+    String name = hasName ? pop() : null;

[Paul Berry, 2017/03/14 14:48:23]

I see what you're doing here, but IMHO it's a little hard to follow.  Consider
doing this instead:

String containingLib = pop();
List<MetadataBuilder> metadata = pop();
if (hasName) {
  library.addPartOf(metadata, containingLib, null);
} else {
  library.addPartOf(metadata, null, containingLib);
}

[ahe, 2017/03/15 14:05:36]

Honestly, I wasn't super happy with it either :-)

+    String uri = hasName ? null : pop();
     List<MetadataBuilder> metadata = pop();
-    library.addPartOf(metadata, name);
+    library.addPartOf(metadata, name, uri);
   }
 
   @override