[dev.ssa] cmd/compile/internal/ssa: Add dummy frontend for testing.

Change-Id: Ica26c0297ac7afeb0b5b668cf5f5cd1667c6cc43
Reviewed-on: https://go-review.googlesource.com/10699
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/export_test.go

@@ -8,3 +8,9 @@ var CheckFunc = checkFunc
 var PrintFunc = printFunc
 var Opt = opt
 var Deadcode = deadcode
+
+type DummyFrontend struct{}
+
+func (d DummyFrontend) StringSym(s string) interface{} {
+	return nil
+}