[infra] Improve efficiency of cloning the Flutter repositories

Sometimes the flutter-analyze builder is flaky because cloning Flutter
exhausts the "short term ls-remote rate limit". "--single-branch"
reduces how many remotes need to be fetched.

Also in this CL:
* Use dart.googlesource.com.
* Apply the same changes to the flutter-frontend builder.

Error example:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8894556948308224672/+/steps/analyze_flutter/0/stdout

Change-Id: I50df781a652602c7dd1b40b938c558b04bd3f86f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127642
Reviewed-by: Jonas Termansen <sortie@google.com>

tools/bots/flutter/analyze_flutter.sh

@@ -16,7 +16,8 @@ cleanup() {
 trap cleanup EXIT HUP INT QUIT TERM PIPE
 cd "$tmpdir"
 
-git clone -vv https://chromium.googlesource.com/external/github.com/flutter/flutter
+git clone --single-branch -vv \
+  https://dart.googlesource.com/external/github.com/flutter/flutter
 
 cd flutter
 

tools/bots/flutter/compile_flutter.sh

@@ -17,7 +17,8 @@ cleanup() {
 trap cleanup EXIT HUP INT QUIT TERM PIPE
 pushd "$tmpdir"
 
-git clone -vv https://chromium.googlesource.com/external/github.com/flutter/flutter
+git clone --single-branch -vv \
+  https://dart.googlesource.com/external/github.com/flutter/flutter
 
 pushd flutter
 bin/flutter config --no-analytics
@@ -26,13 +27,15 @@ patch=$checkout/tools/patches/flutter-engine/${pinned_dart_sdk}.flutter.patch
 if [ -e "$patch" ]; then
   git apply $patch
 fi
+
 bin/flutter update-packages
 popd
 
 # Directly in temp directory again.
 mkdir src
 pushd src
-git clone -vv --depth 1 https://chromium.googlesource.com/external/github.com/flutter/engine flutter
+git clone --single-branch --depth=1 -vv \
+  https://dart.googlesource.com/external/github.com/flutter/engine flutter
 mkdir third_party
 pushd third_party
 ln -s $checkout dart