preemptively fix windows compilation errors

Author: Jorge Aparicio

src/libstd/dynamic_lib.rs

@@ -272,7 +272,9 @@ mod dl {
     use ptr;
     use result::Result;
     use result::Result::{Ok, Err};
+    #[cfg(stage0)]
     use slice::SliceExt;
+    #[cfg(stage0)]
     use str::StrExt;
     use str;
     use string::String;

src/libstd/sys/windows/process2.rs

@@ -128,6 +128,7 @@ impl Process {
         use env::split_paths;
         use mem;
         use iter::IteratorExt;
+        #[cfg(stage0)]
         use str::StrExt;
 
         // To have the spawning semantics of unix/windows stay the same, we need to