Use FormulaInstaller#fetch before #prelude.

Library/Homebrew/cask/installer.rb

@@ -368,8 +368,8 @@ module Cask
             fi.installed_on_request = false
             fi.show_header = true
             fi.verbose = verbose?
-            fi.prelude
             fi.fetch
+            fi.prelude
             fi.install
             fi.finish
           end

Library/Homebrew/cmd/install.rb

@@ -325,8 +325,8 @@ module Homebrew
     fi.build_bottle         = args.build_bottle?
     fi.interactive          = args.interactive?
     fi.git                  = args.git?
-    fi.prelude
     fi.fetch
+    fi.prelude
     fi.install
     fi.finish
   rescue FormulaInstallationAlreadyAttemptedError

Library/Homebrew/cmd/upgrade.rb

@@ -183,7 +183,6 @@ module Homebrew
       fi.installed_as_dependency = tab.installed_as_dependency
       fi.installed_on_request  ||= tab.installed_on_request
     end
-    fi.prelude
 
     upgrade_version = if f.optlinked?
       "#{Keg.new(f.opt_prefix).version} -> #{f.pkg_version}"
@@ -193,6 +192,7 @@ module Homebrew
     oh1 "Upgrading #{Formatter.identifier(f.full_specified_name)} #{upgrade_version} #{fi.options.to_a.join(" ")}"
 
     fi.fetch
+    fi.prelude
 
     # first we unlink the currently active keg for this formula otherwise it is
     # possible for the existing build to interfere with the build we are about to

Library/Homebrew/reinstall.rb

@@ -35,8 +35,8 @@ module Homebrew
       fi.installed_as_dependency = tab.installed_as_dependency
       fi.installed_on_request    = tab.installed_on_request
     end
-    fi.prelude
     fi.fetch
+    fi.prelude
 
     oh1 "Reinstalling #{Formatter.identifier(f.full_name)} #{options.to_a.join " "}"
 

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -176,8 +176,8 @@ RSpec.shared_context "integration test" do
     setup_test_formula(name, content)
     fi = FormulaInstaller.new(Formula[name])
     fi.build_bottle = build_bottle
-    fi.prelude
     fi.fetch
+    fi.prelude
     fi.install
     fi.finish
   end