Add backend tools for extensible iOS apps (#4250)

Instead of embedding Flutter.framework and BuildFlutterApp in the Xcode
project, we store these in flutter.git, where we can reference them.

packages/flutter_tools/bin/xcode_backend.sh

+#!/bin/sh
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+RunCommand() {
+  echo "♦ " $@
+  $@
+  return $?
+}
+
+EchoError() {
+  echo "$@" 1>&2
+}
+
+AssertExists() {
+  RunCommand ls $1
+  if [ $? -ne 0 ]; then
+    EchoError "The path $1 does not exist"
+    exit -1
+  fi
+  return 0
+}
+
+BuildApp() {
+  local project_path=${FLUTTER_APPLICATION_PATH}
+  AssertExists ${project_path}
+
+  local derived_dir=${SOURCE_ROOT}/Flutter
+  RunCommand mkdir -p $derived_dir
+  AssertExists $derived_dir
+
+  RunCommand rm -f ${derived_dir}/Flutter.framework
+  RunCommand rm -f ${derived_dir}/app.so
+  RunCommand rm -f ${derived_dir}/app.flx
+  RunCommand cp -r ${FLUTTER_FRAMEWORK_DIR}/Flutter.framework ${derived_dir}
+  RunCommand pushd ${project_path}
+
+  local local_engine_flag=""
+  if [[ -n "$LOCAL_ENGINE" ]]; then
+    local_engine_flag="--local-engine=$LOCAL_ENGINE"
+  fi
+
+  if [[ $CURRENT_ARCH != "x86_64" ]]; then
+    local interpreter_flag="--interpreter"
+    if [[ "$DART_EXPERIMENTAL_INTERPRETER" != "1" ]]; then
+      interpreter_flag=""
+    fi
+
+    RunCommand ${FLUTTER_ROOT}/bin/flutter --suppress-analytics build aot \
+      --target-platform=ios                                               \
+      --release                                                           \
+      ${interpreter_flag}                                                 \
+      ${local_engine_flag}
+
+    if [[ $? -ne 0 ]]; then
+      EchoError "Failed to build ${project_path}."
+      exit -1
+    fi
+
+    RunCommand cp build/aot/app.so ${derived_dir}/app.so
+  else
+    RunCommand eval "$(echo \"static const int Moo = 88;\" | xcrun clang -x c --shared -o ${derived_dir}/app.so -)"
+  fi
+
+  local precompilation_flag=""
+  if [[ $CURRENT_ARCH != "x86_64" ]] && [[ "$DART_EXPERIMENTAL_INTERPRETER" != "1" ]]; then
+    precompilation_flag="--precompiled"
+  fi
+
+  RunCommand ${FLUTTER_ROOT}/bin/flutter --suppress-analytics build flx \
+    --output-file=${derived_dir}/app.flx                                \
+    ${precompilation_flag}                                              \
+    ${local_engine_flag}                                                \
+
+  if [[ $? -ne 0 ]]; then
+    EchoError "Failed to package ${project_path}."
+    exit -1
+  fi
+
+  RunCommand popd
+
+  echo "Project ${project_path} built and packaged successfully."
+  return 0
+}
+
+BuildApp

packages/flutter_tools/lib/src/cache.dart

@@ -279,6 +279,12 @@ class FlutterEngine {
       if (!dir.existsSync() || allDirty) {
         await _downloadItem('Downloading engine artifacts $dirName...',
           url + dirName + '/artifacts.zip', dir);
+        File frameworkZip = new File(path.join(dir.path, 'Flutter.framework.zip'));
+        if (frameworkZip.existsSync()) {
+          Directory framework = new Directory(path.join(dir.path, 'Flutter.framework'));
+          framework.createSync();
+          os.unzip(frameworkZip, framework);
+        }
       }
     }