Fix non-mac builds by adding "const".

runtime/vm/uri.cc

Index: runtime/vm/uri.cc
diff --git a/runtime/vm/uri.cc b/runtime/vm/uri.cc
index 48b11250f06f8c5cf44dbb87ee9dc5d77d71e22e..b222c5c092a6a3b807de35194b74399694cf8447 100644
--- a/runtime/vm/uri.cc
+++ b/runtime/vm/uri.cc
@@ -370,7 +370,7 @@ static const char* MergePaths(const char* base_path, const char* ref_path) {
   }
 
   // We need to find the last '/' in base_path.
-  char* last_slash = strrchr(base_path, '/');
+  const char* last_slash = strrchr(base_path, '/');
   if (last_slash == NULL) {
     // There is no slash in the base_path.  Return the ref_path unchanged.
     return ref_path;