Add a Travis job with the 'debug-assertions' build flag enabled.

.travis.yml

@@ -55,6 +55,9 @@ matrix:
    - env: GHCVER=8.0.1 SCRIPT=script PARSEC=YES TAGSUFFIX="-parsec"
      os: linux
      sudo: required
+   - env: GHCVER=8.0.1 SCRIPT=script DEBUG_ASSERTIONS=YES TAGSUFFIX="-fdebug-assertions"
+     os: linux
+     sudo: required
    - env: GHCVER=8.0.1 SCRIPT=bootstrap
      sudo: required
      os: linux

travis-script.sh

@@ -143,7 +143,12 @@ fi
 # Needed to work around some bugs in nix-local-build code.
 export CABAL_BUILDDIR="${CABAL_INSTALL_BDIR}"
 
-timed cabal new-build $jobs cabal-install:cabal \
+if [ "x$DEBUG_ASSERTIONS" = "xYES" ]; then
+    CABAL_INSTALL_FLAGS=-fdebug-assertions
+fi
+
+timed cabal new-build $jobs $CABAL_INSTALL_FLAGS \
+                      cabal-install:cabal \
                       cabal-install:integration-tests \
                       cabal-install:integration-tests2 \
                       cabal-install:unit-tests \