Revert "[flutter] Speed-up `linux_web_engine_test` LUCI build by loading caches in parallel"

This reverts commit 260bcbba69c1d9eec2417de1c273b6f26a95bc6f.

Reason for revert: `Linux linux_web_engine_tests` jobs start failing after this change. https://luci-milo.appspot.com/ui/p/flutter/builders/try/Linux%20linux_web_engine_tests/14912/overview ... https://luci-milo.appspot.com/ui/p/flutter/builders/try/Linux%20linux_web_engine_tests/14919/overview

Failure Link: <LINK TO FAILURE>

Original change's description:
> [flutter] Speed-up `linux_web_engine_test` LUCI build by loading caches in parallel
>
> Google-Bug-Id: 520537519
> Change-Id: Ib894795951b556052568e2268042ad72ea002faa
> Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/79500
> Reviewed-by: John McDole <codefu@google.com>
> Commit-Queue: Dmitry Grand <dmgr@google.com>

TBR=flutter-scoped@luci-project-accounts.iam.gserviceaccount.com,jacksongardner@google.com,codefu@google.com,dmgr@google.com

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I2017f82e648c5ff5bde79c93de7224b1e763df30
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/79540
Commit-Queue: Dmitry Grand <dmgr@google.com>
Reviewed-by: Dmitry Grand <dmgr@google.com>
Reviewed-by: John McDole <codefu@google.com>
diff --git a/recipe_modules/cache/__init__.py b/recipe_modules/cache/__init__.py
index ad00b8d..d8b35e9 100644
--- a/recipe_modules/cache/__init__.py
+++ b/recipe_modules/cache/__init__.py
@@ -7,11 +7,9 @@
     'depot_tools/gsutil',
     'recipe_engine/cas',
     'recipe_engine/file',
-    'recipe_engine/futures',
     'recipe_engine/json',
     'recipe_engine/platform',
     'recipe_engine/step',
     'recipe_engine/raw_io',
     'recipe_engine/path',
 ]
-
diff --git a/recipe_modules/cache/api.py b/recipe_modules/cache/api.py
index c5577a0..7168948 100644
--- a/recipe_modules/cache/api.py
+++ b/recipe_modules/cache/api.py
@@ -111,24 +111,14 @@
       ).stdout
       if self._test_data.enabled:
         metadata = metadata or collections.defaultdict(dict)
-      futures = []
       for k, v in metadata['hashes'].items():
         if force:
           self.m.file.rmtree('Clobber local cache: %s' % k, cache_root / k)
         # Mount the cache only if it doesn't exist locally.
         if not self.m.path.exists(cache_root / k):
-          futures.append(
-              self.m.futures.spawn(
-                  self.m.cas.download,
-                  'Mounting %s with hash %s' % (k, v),
-                  v,
-                  cache_root / k,
-              )
+          self.m.cas.download(
+              'Mounting %s with hash %s' % (k, v), v, cache_root / k
           )
-      if futures:
-        self.m.futures.wait(futures)
-        for future in futures:
-          future.result()
 
   def should_force_mount(self, mount_path):
     """Determines if a cache should be force mounted.
diff --git a/recipe_modules/cache/tests/refresh.expected/basic.json b/recipe_modules/cache/tests/refresh.expected/basic.json
index 5ee97b1..9b49ce3 100644
--- a/recipe_modules/cache/tests/refresh.expected/basic.json
+++ b/recipe_modules/cache/tests/refresh.expected/basic.json
@@ -219,22 +219,6 @@
   },
   {
     "cmd": [
-      "vpython3",
-      "-u",
-      "RECIPE_MODULE[recipe_engine::file]/resources/fileutil.py",
-      "--json-output",
-      "/path/to/tmp/json",
-      "rmtree",
-      "[CACHE]/git"
-    ],
-    "infra_step": true,
-    "name": "Mount caches.Clobber local cache: git",
-    "~followup_annotations": [
-      "@@@STEP_NEST_LEVEL@1@@@"
-    ]
-  },
-  {
-    "cmd": [
       "[START_DIR]/cipd_tool/infra/tools/luci/cas/33f9d887e5b8aeaaf9d65506acccfa8da2c480712e534a23a79e92c342c44bee/cas",
       "download",
       "-cas-instance",
@@ -252,6 +236,22 @@
   },
   {
     "cmd": [
+      "vpython3",
+      "-u",
+      "RECIPE_MODULE[recipe_engine::file]/resources/fileutil.py",
+      "--json-output",
+      "/path/to/tmp/json",
+      "rmtree",
+      "[CACHE]/git"
+    ],
+    "infra_step": true,
+    "name": "Mount caches.Clobber local cache: git",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
+  },
+  {
+    "cmd": [
       "[START_DIR]/cipd_tool/infra/tools/luci/cas/33f9d887e5b8aeaaf9d65506acccfa8da2c480712e534a23a79e92c342c44bee/cas",
       "download",
       "-cas-instance",
diff --git a/recipe_modules/cache/tests/refresh.expected/no_cache_file.json b/recipe_modules/cache/tests/refresh.expected/no_cache_file.json
index 5f3b4ba..bfed5d0 100644
--- a/recipe_modules/cache/tests/refresh.expected/no_cache_file.json
+++ b/recipe_modules/cache/tests/refresh.expected/no_cache_file.json
@@ -201,22 +201,6 @@
   },
   {
     "cmd": [
-      "vpython3",
-      "-u",
-      "RECIPE_MODULE[recipe_engine::file]/resources/fileutil.py",
-      "--json-output",
-      "/path/to/tmp/json",
-      "rmtree",
-      "[CACHE]/git"
-    ],
-    "infra_step": true,
-    "name": "Mount caches.Clobber local cache: git",
-    "~followup_annotations": [
-      "@@@STEP_NEST_LEVEL@1@@@"
-    ]
-  },
-  {
-    "cmd": [
       "[START_DIR]/cipd_tool/infra/tools/luci/cas/33f9d887e5b8aeaaf9d65506acccfa8da2c480712e534a23a79e92c342c44bee/cas",
       "download",
       "-cas-instance",
@@ -234,6 +218,22 @@
   },
   {
     "cmd": [
+      "vpython3",
+      "-u",
+      "RECIPE_MODULE[recipe_engine::file]/resources/fileutil.py",
+      "--json-output",
+      "/path/to/tmp/json",
+      "rmtree",
+      "[CACHE]/git"
+    ],
+    "infra_step": true,
+    "name": "Mount caches.Clobber local cache: git",
+    "~followup_annotations": [
+      "@@@STEP_NEST_LEVEL@1@@@"
+    ]
+  },
+  {
+    "cmd": [
       "[START_DIR]/cipd_tool/infra/tools/luci/cas/33f9d887e5b8aeaaf9d65506acccfa8da2c480712e534a23a79e92c342c44bee/cas",
       "download",
       "-cas-instance",