Add support for LEGACY version

Bug: 106549
Change-Id: I90461e0cbf60064130e4f80534929176ce00b796
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidlbolt/+/740608
Reviewed-by: Alex Zaslavsky <azaslavsky@google.com>
diff --git a/backend/server.go b/backend/server.go
index cf57363..28d1718 100644
--- a/backend/server.go
+++ b/backend/server.go
@@ -172,7 +172,7 @@
 }
 
 // versionSelectionRegexp is used to validate Options.VersionSelection.
-var versionSelectionRegexp = regexp.MustCompile(`^\s*([a-z][a-z0-9_]*:([0-9]+|HEAD)\s*)*$`)
+var versionSelectionRegexp = regexp.MustCompile(`^\s*([a-z][a-z0-9_]*:([0-9]+|HEAD|LEGACY)\s*)*$`)
 
 // An annotation is a message targeted at a specific location of a file.
 type annotation struct {
diff --git a/frontend/src/elm/Mode.elm b/frontend/src/elm/Mode.elm
index fb21bf1..8d0210f 100644
--- a/frontend/src/elm/Mode.elm
+++ b/frontend/src/elm/Mode.elm
@@ -170,7 +170,7 @@
     let
         versionSelectionRegex =
             Maybe.withDefault Regex.never <|
-                Regex.fromString "^\\s*([a-z][a-z0-9_]*:([0-9]+|HEAD)\\s*)*$"
+                Regex.fromString "^\\s*([a-z][a-z0-9_]*:([0-9]+|HEAD|LEGACY)\\s*)*$"
 
         entries =
             case input of