Disable isolate_dependency_tracked in build driver for .isolate files.

build/isolate.gypi

Index: build/isolate.gypi
diff --git a/build/isolate.gypi b/build/isolate.gypi
index f31dce9a8091296b915bca7c5cd80949bd78cf30..8f5d71a8604073bade347cb2783e82c39c9915a1 100644
--- a/build/isolate.gypi
+++ b/build/isolate.gypi
@@ -40,8 +40,18 @@
       'rule_name': 'isolate',
       'extension': 'isolate',
       'inputs': [
-        '<@(isolate_dependency_tracked)',
         '<(RULE_INPUT_PATH)',
+        # Disable file tracking by the build driver for now. This means the
+        # project must have the proper build-time dependency for their runtime
+        # dependency. This improves the runtime of the build driver since it
+        # doesn't have to stat() all these files.
+        #
+        # More importantly, it means that even if a isolate_dependency_tracked
+        # file is missing, for example if a file was deleted and the .isolate
+        # file was not updated, that won't break the build, especially in the
+        # case where foo_tests_run is not built! This should be reenabled once
+        # the switch-over to running tests on Swarm is completed.
+        #'<@(isolate_dependency_tracked)',
       ],
       'outputs': [
         '<(PRODUCT_DIR)/<(RULE_INPUT_ROOT).isolated',