Add buck build soyc to output GWT split point report

The GWT compiler can output a report (or "story of your compile"),
describing the size of the JavaScript and which source classes
contributed to the overall download size. This is useful for
optimizing code splits in Dispatcher, and generally understanding
the impact of linking code together.

Only the "Split Point Report" is created.
The "Compiler Metrics" are not output.

The report is very slow to create, so it is not done by default.

Change-Id: I6ce86d65ee5f3e4bdba573d1f777ccc59767341d

.buckconfig

@@ -11,6 +11,7 @@
   gerrit = //:gerrit
   release = //:release
   safari = //:safari
+  soyc = //gerrit-gwtui:ui_soyc
   withdocs = //:withdocs
 
 [buildfile]

.gitignore

@@ -15,6 +15,7 @@
 /.buckd
 /buck-cache
 /buck-out
+/extras
 /local.properties
 *.pyc
 /gwt-unitCache

Documentation/dev-buck.txt

@@ -231,6 +231,26 @@ The WAR file will be placed in:
   buck-out/gen/withdocs.war
 ----
 
+[[soyc]]
+=== GWT Compile Report
+
+The GWT compiler can output a compile report (or "story of your compile"),
+describing the size of the JavaScript and which source classes contributed
+to the overall download size.
+
+----
+  buck build soyc
+----
+
+The report will be written as an HTML page to the extras directory, and
+can be opened and viewed in any web browser:
+
+----
+  extras/gerrit_ui/soycReport/compile-report/index.html
+----
+
+Only the "Split Point Report" is created, "Compiler Metrics" are not output.
+
 [[release]]
 === Gerrit Release WAR File
 

gerrit-gwtui/BUCK

@@ -36,6 +36,17 @@ gwt_binary(
   vm_args = GWT_JVM_ARGS,
 )
 
+gwt_binary(
+  name = 'ui_soyc',
+  modules = [MODULE],
+  module_deps = [':ui_module'],
+  deps = DEPS + [':ui_dbg'],
+  local_workers = cpu_count(),
+  strict = True,
+  experimental_args = GWT_COMPILER_ARGS + ['-compileReport'],
+  vm_args = GWT_JVM_ARGS,
+)
+
 gwt_binary(
   name = 'ui_dbg',
   modules = [MODULE],