Force the compilation mode for tests to fastbuild.

We can't use 'opt', because that trims out certain features needed by
the unit testing framework. Using 'dbg' also doesn't make sense, because
the time traveling debugger isn't usable in this context.

elm/def.bzl

@@ -8,6 +8,7 @@ _TOOLCHAIN = "@com_github_edschouten_rules_elm//elm:toolchain"
 
 def _do_elm_make(
         ctx,
+        compilation_mode,
         main,
         deps,
         additional_source_directories,
@@ -55,7 +56,7 @@ def _do_elm_make(
         executable = "python",
         arguments = [
             ctx.files._compile[0].path,
-            ctx.var["COMPILATION_MODE"],
+            compilation_mode,
             toolchain.elm.files.to_list()[0].path,
             elm_json.path,
             main.path,
@@ -69,11 +70,13 @@ def _do_elm_make(
 
 def _elm_binary_impl(ctx):
     js_file = ctx.actions.declare_file(ctx.attr.name + ".js")
-    if ctx.var["COMPILATION_MODE"] == "opt":
+    compilation_mode = ctx.var["COMPILATION_MODE"]
+    if compilation_mode == "opt":
         # Step 1: Compile the Elm code.
         js1_file = ctx.actions.declare_file(ctx.attr.name + ".1.js")
         _do_elm_make(
             ctx,
+            compilation_mode,
             ctx.files.main[0],
             ctx.attr.deps,
             [],
@@ -117,6 +120,7 @@ def _elm_binary_impl(ctx):
         # Don't attempt to compress the code after building.
         _do_elm_make(
             ctx,
+            compilation_mode,
             ctx.files.main[0],
             ctx.attr.deps,
             [],
@@ -216,6 +220,7 @@ def _elm_test_impl(ctx):
     elmi_file = ctx.actions.declare_file(elmi_filename)
     _do_elm_make(
         ctx,
+        "fastbuild",
         ctx.files.main[0],
         ctx.attr.deps,
         [],
@@ -246,6 +251,7 @@ def _elm_test_impl(ctx):
     js_file = ctx.actions.declare_file(ctx.attr.name + ".js")
     _do_elm_make(
         ctx,
+        "fastbuild",
         main_file,
         ctx.attr.deps + [ctx.attr._node_test_runner],
         [ctx.files.main[0].dirname],