bazel: "warning: could not find a JDK 8 bootclasspath in external/embedded_jdk, falling back to --release" on Bazel 0.19.0rc6 and master

On Bazel 0.19.0rc6 I’m seeing this warning during build of gerrit and gerrit plugins:

  $ bazel build reviewers
INFO: Build options have changed, discarding analysis cache.
INFO: Analysed target //:reviewers (36 packages loaded, 527 targets configured).
INFO: Found 1 target...
INFO: From SkylarkAction external/bazel_tools/tools/jdk/platformclasspath.jar [for host]:
warning: could not find a JDK 8 bootclasspath in external/embedded_jdk, falling back to --release
INFO: From GWT compiling reviewers__gwt_application.zip [for host]:
Compiling module com.googlesource.gerrit.plugins.reviewers.ReviewersForm
[...]

I’m seeing the same warning on Bazel master as well (d48e0e27600deeed9eab01d6e42ab4b6e715d7a6). This seems to be a regression, as all is fine on Bazel 0.18.0:

  $ bazel version
Build label: 0.18.0

  $ bazel build reviewers
Starting local Bazel server and connecting to it...
INFO: Analysed target //:reviewers (36 packages loaded).
INFO: Found 1 target...
INFO: From GWT compiling reviewers__gwt_application.zip [for host]:
Compiling module com.googlesource.gerrit.plugins.reviewers.ReviewersForm
   Compiling 5 permutations
      Compiling permutation 0...
      Compiling permutation 1...
      Compiling permutation 2...
      Compiling permutation 3...
      Compiling permutation 4...
   Compile of permutations succeeded
   Compilation succeeded -- 24.064s
Linking into /home/davido/.cache/bazel/_bazel_davido/6bfabb416f8d36020b7be6ab9749326b/sandbox/linux-sandbox/7/execroot/reviewers/bazel-out/host/bin/reviewers__gwt_application.zip.gwt_output/reviewers
   Link succeeded
   Linking succeeded -- 0.188s
Target //:reviewers up-to-date:
  bazel-genfiles/reviewers.jar
INFO: Elapsed time: 44.457s, Critical Path: 33.80s
INFO: 13 processes: 10 linux-sandbox, 3 worker.
INFO: Build completed successfully, 15 total actions

//CC @cushon.