Spun off from  bug 631575  comment 10 ( http://crbug.com/631575#c10  )

$ gn refs out/Default "third_party/icu/source/common/unicode/unistr.h" --all
The input matches no targets, configs, or files.

As a result, when the only change in ICU is a header file, trybot (that uses gn refs to determine what targets to recompile for a given CL) does not recompile anything. 

$ gn refs out/Default "third_party/icu/source/common/unistr.cpp" --all

//:All
//:blink_tests
//:both_gn_and_gyp
//:chromium_builder_asan
//:chromium_builder_perf
//:gn_all
//:gn_mojo_targets
//:gn_only
//:gn_visibility
//apps:apps
..... snip ......

unistr.cpp is listed in source in icu/BUILD.gn but none of public (e.g. icu/source/common/unicode/unistr.h) and private header files (e.g. icu/source/common/umutex.h) are listed in BUILD.gn. Instead, icu/BUILD.gn has include_dirs set to ["source/common", "source/i18n"].

If all the private headers are listed in sources[] and all the public headers are listed in public[],  'gn refs ...' will match a header file. 

When I do that with one public header file and one private header file for icuuc target, 'gn check' does not complain and 'gn refs' works as expected. 

Let me see if gn check is still ok with all the header files listed (private headers in sources[] and public headers in public[]).