Bump DevTools DEP to 2.20.1 and modify the devtools/update.sh script to support passing an additional argument to the DevTools build script.

Change-Id: I176f72c4d2e562f923d174734cbd2c4f8fc19b83
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279335
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

DEPS

@@ -126,7 +126,7 @@ vars = {
   "dart_style_rev": "d0c6b1da52c9f80679a7aaa05e5935ef5c6bb52f", # manually rev'd
 
   "dartdoc_rev": "ed56883d7b3c31aea3faa27232c3bffc2c97aaa4",
-  "devtools_rev": "dd3fade2bd2ef74fc6102e56333a48f0efc594d3",
+  "devtools_rev": "c7a81f64dabb4b9f2cae0cf2c9411ef8bb805191",
   "ffi_rev": "2a56c2a9a87122c057168874bde384debbb806b0",
   "file_rev": "b768f79dcd104a5feabafab47101c4355b71cd8f",
   "fixnum_rev": "71f0d4d16054e6be7d8e22bdb3b082b9f82061be",

third_party/devtools/update.sh

@@ -24,8 +24,9 @@ git clone git@github.com:flutter/devtools.git
 cd devtools
 git checkout -b cipd_release $1
 
-./tool/update_flutter_sdk.sh
-./tool/build_release.sh
+# If $2 equals "--no-update-flutter", the build_release.sh script will build
+# DevTools from the Flutter SDK that already exists on the user's path.
+./tool/build_release.sh $2
 
 # Copy the build output as well as the devtools packages needed
 # to serve from DDS.