librustc: Stop reexporting the standard modules from prelude.

src/libsyntax/ext/pipes/ast_builder.rs

@@ -19,6 +19,9 @@ use ast::ident;
 use ast;
 use codemap::span;
 
+use core::str;
+use core::vec;
+
 // Transitional reexports so qquote can find the paths it is looking for
 mod syntax {
     pub use ext;