Revert "Revert "next try for "don't abort when given unmatched '}' with 'start-condition stack underflow'. This fixes #751"""

This reverts commit b669d3d2e8.

(cherry picked from commit 5d8b7eb3e1)

tests/lexer.nix

@@ -0,0 +1,2 @@
+let const = a: "const"; in
+''${ const { x = "q"; }}''

tests/lexer.sh

@@ -0,0 +1,3 @@
+source common.sh
+
+nix-instantiate --eval ./lexer.nix

tests/local.mk

@@ -11,7 +11,7 @@ nix_tests = \
   binary-patching.sh timeout.sh secure-drv-outputs.sh nix-channel.sh \
   multiple-outputs.sh import-derivation.sh fetchurl.sh optimise-store.sh \
   binary-cache.sh nix-profile.sh repair.sh dump-db.sh case-hack.sh \
-  check-reqs.sh pass-as-file.sh tarball.sh
+  check-reqs.sh pass-as-file.sh tarball.sh lexer.sh
   # parallel.sh
 
 install-tests += $(foreach x, $(nix_tests), tests/$(x))