Windows: Fix use of IsAbs check

Signed-off-by: John Howard <jhoward@microsoft.com>

system/filesys.go

@@ -4,6 +4,7 @@ package system
 
 import (
 	"os"
+	"path/filepath"
 )
 
 // MkdirAll creates a directory named path along with any necessary parents,
@@ -11,3 +12,8 @@ import (
 func MkdirAll(path string, perm os.FileMode) error {
 	return os.MkdirAll(path, perm)
 }
+
+// IsAbs is a platform-specific wrapper for filepath.IsAbs.
+func IsAbs(path string) bool {
+	return filepath.IsAbs(path)
+}