Bisected this back to https://chromium-review.googlesource.com/1255046 - that CL introduced an missing dependency from netstack to zircon.ethernet, so perhaps we're missing something related to that.

Release monolithic build continued to work (otherwise the roll couldn't have landed), while Release component build is also broken.

Successful (pre-roll) run:

[00007.636] pkgsvr: 2018/10/04 00:43:08 pkgfs: importing package from "4ea6b9a1eea528b103a9d6e6f2c45f74da123d239d4fc51a87d326449e7b7080"
[00007.654] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "d34b4d7a2808e3d1ff897ecaef097be550fd2f1d779a707ea794719122ed9ee1"
[00007.657] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "aee921acbbb6c7a45f4375ceed4a5c6e680ae8780ad58334325034b8570d65c0"
[00007.658] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "a39749df3e4aceb5eb75019d5333f62291daf0c90ad028dddadaa85c678c2e4d"
[00007.658] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "b52c790b33797fbc4e094b6552aa2d122f9bc81c6a33b43fd5f4341ae1e68dbe"
[00007.658] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "5e2cc4ddfbbbc60252d12e1b3559c33eb61e09c687c46981ce4999e79c2da930"
[00007.658] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "47fe4c5f91f48b0c3e4b867abed29bf96b8afe3b8b75241fceb1b1cb93b02bf1"
[00007.658] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "51a6c6ff3430a33c2a42662ed52cd0b0551baf8cf58111c1179330c33231f29d"
[00007.659] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "2f8d9068841ef78b0a4dfe3f65e3907a70cd698f51b1ac1d4d1730016399f366"
[00007.661] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "5ad7936c577ba469c0007475c8dc14f7b83673945dc31e8cc5983c8a3604e357"
[00007.661] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "7382b93c96c7c88d8ce801203ac6dec2579e26ee198690e5f3ef625000a5198a"
[00007.661] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "23b08f9397cb6e4f51a5e5e62826e1cf449648c1934767f151969f6afb7ba8ac"
[00007.661] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "e14b9dd48a373835875eca092a368c51369199493a6ea89c1d957b98282823f8"
[00007.662] pkgsvr: 2018/10/04 00:43:08 pkgfs: blob already present for {cronet_unittests 0}: "e88d083a786992e9c7d9d8ce8055d95c1ad9123c3b3b468ec1b15ec8440be50b"
[00007.663] pkgsvr: 2018/10/04 00:43:08 pkgfs: skipping bad package entry: ""
[00007.667] pkgsvr: 2018/10/04 00:43:08 pkgfs: asking amber to fetch 2306 blobs for {cronet_unittests 0}
[00009.416] pkgsvr: 2018/10/04 00:43:09 pkgfs: index: Amber notification error: ErrPeerClosed: zx.Channel.Write
[00009.416] pkgsvr: 2018/10/04 00:43:09 pkgfs: index: Amber notification error: ErrPeerClosed: zx.Channel.Write
2018-10-03 17:43:10,696:DEBUG:root:running 'rm /data/cronet_unittests.far'.
2018-10-03 17:43:10,697:DEBUG:ssh:ssh exec: ssh -F /usr/local/google/home/wez/Projects/git-cr-fuchsia/src/out/gnRelease/ssh_config localhost -p 49169 rm /data/cronet_unittests.far
2018-10-03 17:43:10,829:INFO:root:Running application.
2018-10-03 17:43:10,830:DEBUG:root:running (non-blocking) 'run cronet_unittests --test-launcher-retry-limit=0 --test-launcher-jobs=4'.
2018-10-03 17:43:10,830:DEBUG:ssh:ssh -F /usr/local/google/home/wez/Projects/git-cr-fuchsia/src/out/gnRelease/ssh_config localhost -p 49169 -- run cronet_unittests --test-launcher-retry-limit=0 --test-launcher-jobs=4
2018-10-03 17:43:10,837:DEBUG:root:Symbols: libnet.so -> lib/libnet.so
2018-10-03 17:43:10,843:DEBUG:root:Symbols: libnet_with_v8.so -> lib/libnet_with_v8.so
2018-10-03 17:43:10,846:DEBUG:root:Symbols: libgin.so -> lib/libgin.so
2018-10-03 17:43:10,847:DEBUG:root:Symbols: cronet_unittests__exec -> cronet_unittests__exec
2018-10-03 17:43:10,849:DEBUG:root:Symbols: libprotobuf_lite.so -> lib/libprotobuf_lite.so
2018-10-03 17:43:10,849:DEBUG:root:Symbols: libbase.so -> lib/libbase.so
2018-10-03 17:43:10,856:DEBUG:root:Symbols: liburl.so -> lib/liburl.so
2018-10-03 17:43:10,858:DEBUG:root:Symbols: libc++.so -> lib/libc++.so
2018-10-03 17:43:10,859:DEBUG:root:Symbols: libicuuc_cr.so -> lib/libicuuc_cr.so
2018-10-03 17:43:10,862:DEBUG:root:Symbols: libbase_i18n.so -> lib/libbase_i18n.so
2018-10-03 17:43:10,867:DEBUG:root:Symbols: libcrcrypto.so -> lib/libcrcrypto.so
2018-10-03 17:43:10,876:DEBUG:root:Symbols: libprefs.so -> lib/libprefs.so
2018-10-03 17:43:10,882:DEBUG:root:Symbols: libv8_libbase.so -> lib/libv8_libbase.so
2018-10-03 17:43:10,884:DEBUG:root:Symbols: libicui18n_cr.so -> lib/libicui18n_cr.so
2018-10-03 17:43:10,884:DEBUG:root:Symbols: libboringssl.so -> lib/libboringssl.so
2018-10-03 17:43:10,884:DEBUG:root:Symbols: libv8.so -> lib/libv8.so
[00009.742] pkgsvr: 2018/10/04 00:43:10 pkgfs:unsupported(/packages/cronet_unittests/0): dir unlink "cronet_unittests__exec.log"

cronet_unittests.far was 100499456 bytes.

Unsuccessful (post-roll) run:

[00013.179] pkgsvr: 2018/10/04 00:54:23 pkgfs: importing package from "257e7d45030e83869dfe915f13f98ab616f67d87710e341ffea0e5d67885be0a"
[00013.188] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "fb66fcb84e135a29488ee572d3829b47bc47c6dd30dfb0d4872a12946d22b9a9"
[00013.188] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "51a6c6ff3430a33c2a42662ed52cd0b0551baf8cf58111c1179330c33231f29d"
[00013.189] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "2f8d9068841ef78b0a4dfe3f65e3907a70cd698f51b1ac1d4d1730016399f366"
[00013.189] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "a58350d5c7f99441401d1fee101eb0b68499852c153a2735690bd93e56086b28"
[00013.189] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "765988e31584291468fa927f67d20ceead11685553e67fe928661643ca6f94c7"
[00013.189] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "23b08f9397cb6e4f51a5e5e62826e1cf449648c1934767f151969f6afb7ba8ac"
[00013.189] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "e14b9dd48a373835875eca092a368c51369199493a6ea89c1d957b98282823f8"
[00013.190] pkgsvr: 2018/10/04 00:54:23 pkgfs: blob already present for {cronet_unittests 0}: "e88d083a786992e9c7d9d8ce8055d95c1ad9123c3b3b468ec1b15ec8440be50b"
[00013.191] pkgsvr: 2018/10/04 00:54:23 pkgfs: skipping bad package entry: ""
[00013.201] pkgsvr: 2018/10/04 00:54:23 pkgfs: asking amber to fetch 2311 blobs for {cronet_unittests 0}
2018-10-03 17:54:27,040:DEBUG:root:running 'rm /data/cronet_unittests.far'.
2018-10-03 17:54:27,040:DEBUG:ssh:ssh exec: ssh -F /usr/local/google/home/wez/Projects/git-cr-fuchsia/src/out/gnRelease/ssh_config localhost -p 56051 rm /data/cronet_unittests.far
2018-10-03 17:54:27,174:INFO:root:Running application.
2018-10-03 17:54:27,174:DEBUG:root:running (non-blocking) 'run cronet_unittests --test-launcher-retry-limit=0 --test-launcher-jobs=4'.
2018-10-03 17:54:27,175:DEBUG:ssh:ssh -F /usr/local/google/home/wez/Projects/git-cr-fuchsia/src/out/gnRelease/ssh_config localhost -p 56051 -- run cronet_unittests --test-launcher-retry-limit=0 --test-launcher-jobs=4
2018-10-03 17:54:27,181:DEBUG:root:Symbols: libnet.so -> lib/libnet.so
2018-10-03 17:54:27,189:DEBUG:root:Symbols: libnet_with_v8.so -> lib/libnet_with_v8.so
2018-10-03 17:54:27,192:DEBUG:root:Symbols: libgin.so -> lib/libgin.so
2018-10-03 17:54:27,194:DEBUG:root:Symbols: cronet_unittests__exec -> cronet_unittests__exec
2018-10-03 17:54:27,195:DEBUG:root:Symbols: libprotobuf_lite.so -> lib/libprotobuf_lite.so
2018-10-03 17:54:27,196:DEBUG:root:Symbols: libbase.so -> lib/libbase.so
2018-10-03 17:54:27,204:DEBUG:root:Symbols: liburl.so -> lib/liburl.so
2018-10-03 17:54:27,206:DEBUG:root:Symbols: libc++.so -> lib/libc++.so
2018-10-03 17:54:27,208:DEBUG:root:Symbols: libicuuc_cr.so -> lib/libicuuc_cr.so
2018-10-03 17:54:27,210:DEBUG:root:Symbols: libbase_i18n.so -> lib/libbase_i18n.so
2018-10-03 17:54:27,214:DEBUG:root:Symbols: libcrcrypto.so -> lib/libcrcrypto.so
2018-10-03 17:54:27,218:DEBUG:root:Symbols: libprefs.so -> lib/libprefs.so
2018-10-03 17:54:27,221:DEBUG:root:Symbols: libv8_libbase.so -> lib/libv8_libbase.so
2018-10-03 17:54:27,223:DEBUG:root:Symbols: libicui18n_cr.so -> lib/libicui18n_cr.so
2018-10-03 17:54:27,223:DEBUG:root:Symbols: libboringssl.so -> lib/libboringssl.so
2018-10-03 17:54:27,223:DEBUG:root:Symbols: libv8.so -> lib/libv8.so
[00015.247] [ERROR:garnet/bin/appmgr/root_loader.cc(86)] Could not load url: file://cronet_unittests

cronet_unittests.far was 100524032 bytes.

Note to tomorrow self: doesn't repro locally

Actually it reproes when I run the build from the right output directory. Doh.

The NextAction date has arrived: 2018-10-04

A further SDK roll fixed it.
My job here is done.