Build engine for iOS on presubmit (flutter#8011)

* build the ios engine on Cirrus

.cirrus.yml

@@ -45,6 +45,39 @@ task:
         mkdir javadoc_tmp
         ./flutter/tools/gen_javadoc.py --out-dir javadoc_tmp
 
+task:
+  name: build_ios
+  use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
+  osx_instance:
+    image: high-sierra-xcode-9.4.1
+  env:
+    CIRRUS_WORKING_DIR: "/tmp/github_repo"
+    ENGINE_PATH: "/tmp/engine"
+    DEPOT_TOOLS: "/tmp/depot_tools"
+    PATH: "$DEPOT_TOOLS:$PATH"
+  depot_tools_script:
+    git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git $DEPOT_TOOLS
+  # jazzy_script:
+  #   sudo gem install [email protected]
+  gclient_sync_script: |
+    mkdir -p $ENGINE_PATH/src
+    echo 'solutions = [{"managed": False,"name": "src/flutter","url": "[email protected]:flutter/engine.git","custom_deps": {},"deps_file": "DEPS","safesync_url": "",},]' > $ENGINE_PATH/.gclient
+    cd $ENGINE_PATH/src
+    rm -rf flutter
+    rm -rf out
+    mv $CIRRUS_WORKING_DIR flutter
+    gclient sync
+  compile_host_script: |
+    cd $ENGINE_PATH/src
+    ./flutter/tools/gn --ios --unoptimized
+    ninja -C out/ios_debug_unopt
+    # TODO(dnfield): when we can install jazzy properly, we should do this.
+    # AFAICT we can't because of Xcode version.
+    # mkdir objcdoc_tmp
+    # pushd flutter
+    # ./tools/gen_objcdoc.sh ../objcdoc_tmp
+    # popd
+
 format_and_dart_test_task:
   container:
     image: gcr.io/flutter-cirrus/build-engine-image:latest

tools/gen_objcdoc.sh

@@ -24,15 +24,6 @@ if [[ -n "${GEM_HOME}" ]]
     PATH="${GEM_HOME}/bin:$PATH"
 fi
 
-# Jazzy 0.8.x, used on chrome infra, expects --no-download-badge
-# Jazzy 0.9.x, used on LUCI, doesn't support that flag.
-noDownloadBadge=""
-if [[ -z "$LUCI_CONTEXT" ]]
-  then
-    noDownloadBadge="--no-download-badge"
-fi
-
-
 # Use iPhoneSimulator SDK
 # See: https://github.com/realm/jazzy/issues/791
 jazzy \
@@ -47,5 +38,4 @@ jazzy \
   --xcodebuild-arguments --objc,shell/platform/darwin/ios/framework/Headers/Flutter.h,--,-x,objective-c,-isysroot,"$(xcrun --show-sdk-path --sdk iphonesimulator)",-I,"$(pwd)"\
   --module Flutter\
   --root-url https://docs.flutter.io/objc/\
-  --output "$1"\
-  $noDownloadBadge
+  --output "$1"