Fix HTTP links in URI docs.

lib/dartdoc/inline_parser.dart

Index: lib/dartdoc/inline_parser.dart
diff --git a/lib/dartdoc/inline_parser.dart b/lib/dartdoc/inline_parser.dart
index 69c63edc0eff64441094a64590c6a2d8002bf8b8..dfefc2ae82af78945076b280516cc54a34bac132 100644
--- a/lib/dartdoc/inline_parser.dart
+++ b/lib/dartdoc/inline_parser.dart
@@ -288,6 +288,9 @@ class LinkSyntax extends TagSyntax {
         id = parser.source.substring(state.startPos + 1, parser.pos);
       }
 
+      // References are case-insensitive.
+      id = id.toLowerCase();
+
       // Look up the link.
       final link = parser.document.refLinks[id];
       // If it's an unknown link just emit plaintext.