[core] Avoid implicit downcast from dynamic

Add a static type to the variable declaration so it isn't inferred
as dynamic causing multiple implicit downcasts downstream.

CoreLibraryReviewExempt: Trivial change.
Change-Id: Ibef1d4666f5a6a5bbab363195c4b4dea66e8eac3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/328741
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

sdk/lib/core/uri.dart

@@ -4206,7 +4206,7 @@ List<Uint8List> _createTables() {
   }
 
   // Create the transitions for each state.
-  var b;
+  Uint8List b;
 
   // Validate as path, if it is a scheme, we handle it later.
   b = build(_uriStart, schemeOrPath | notSimple);