Experiment with Travis's build stages.

See haskell#4556.

.travis.yml

@@ -114,13 +114,31 @@ install:
  # We intentionally do not install anything before trying to build Cabal because
  # it should build with each supported GHC version out-of-the-box.
 
-# Here starts the actual work to be performed for the package under test; any
-# command which exits with a non-zero exit code causes the build to fail. Using
-# ./dist/setup/setup here instead of cabal-install to avoid breakage when the
-# build config format changed.
-script:
- - rm -rf dist-newstyle
- - ./travis-${SCRIPT}.sh -j2
+# All jobs except 'meta' have two stages; the first stage is used to
+# warm up the cache. This is mostly useful when a new compiler gets
+# added to the build matrix: work done installing dependencies is not
+# wasted when we exceed the time limit for jobs.
+jobs:
+  include:
+    - stage: prepare cache
+      env:
+        # All except 'meta' basically.
+        - SCRIPT=script
+        - SCRIPT=solver-debug-flags
+        - SCRIPT=bootstrap
+        - SCRIPT=stack
+      # TODO: Don't build Cabal and hackage-security here, they'll be
+      # rebuilt anyway.
+      script: cabal new-build --only-dependencies cabal-install cabal-tests
+    - stage: test
+      # Here starts the actual work to be performed for the package
+      # under test; any command which exits with a non-zero exit code
+      # causes the build to fail. Using ./dist/setup/setup here
+      # instead of cabal-install to avoid breakage when the build
+      # config format changed.
+      script:
+        - rm -rf dist-newstyle
+        - ./travis-${SCRIPT}.sh -j2
 
 cache:
  directories: