update AstBuilder to support part of <uri>

pkg/analyzer/lib/src/fasta/ast_builder.dart

Index: pkg/analyzer/lib/src/fasta/ast_builder.dart
diff --git a/pkg/analyzer/lib/src/fasta/ast_builder.dart b/pkg/analyzer/lib/src/fasta/ast_builder.dart
index f61af91664debeb47f8a0346e893212c739d5f14..7580511ad0e7d1d44dadacb5129acc57205adbaa 100644
--- a/pkg/analyzer/lib/src/fasta/ast_builder.dart
+++ b/pkg/analyzer/lib/src/fasta/ast_builder.dart
@@ -1421,9 +1421,14 @@ class AstBuilder extends ScopeListener {
   @override
   void endPartOf(Token partKeyword, Token semicolon, bool hasName) {
     debugEvent("PartOf");
-    List<SimpleIdentifier> libraryName = pop();
-    var name = ast.libraryIdentifier(libraryName);
-    StringLiteral uri = null; // TODO(paulberry)
+    var libraryNameOrUri = pop();
+    LibraryIdentifier name;
+    StringLiteral uri;
+    if (libraryNameOrUri is StringLiteral) {
+      uri = libraryNameOrUri;
+    } else {
+      name = ast.libraryIdentifier(libraryNameOrUri);
+    }
     // TODO(paulberry,ahe): seems hacky.  It would be nice if the parser passed
     // in a reference to the "of" keyword.
     var ofKeyword = partKeyword.next;