Make the user specify the path to the flutter repo in Xcode

sky/build/PackagerInvoke

@@ -36,8 +36,6 @@ PackageProject() {
     exit -1
   fi
 
-  local pub_path="$(which pub)"
-
   AssertExists $1
   local project_path=$1
 
@@ -61,7 +59,7 @@ PackageProject() {
   # containing the pubspec
   RunCommand pushd ${project_path}
 
-  RunCommand ${pub_path} run sky_tools build                                   \
+  RunCommand ${FLUTTER_PATH}/bin/flutter build                                 \
       --asset-base ${icons_path}                                               \
       --main ${dart_main}                                                      \
       --output-file ${derived_dir}/app.flx                                     \

sky/build/sdk_xcode_harness/FlutterApplication.xcodeproj/project.pbxproj

@@ -502,6 +502,7 @@
 				DYLIB_CURRENT_VERSION = 1;
 				DYLIB_INSTALL_NAME_BASE = "@rpath";
 				FLUTTER_APPLICATION_PATH = path/to/application;
+				FLUTTER_PATH = path/to/flutter/repo;
 				INFOPLIST_FILE = FlutterApplication/Info.plist;
 				INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
 				IPHONEOS_DEPLOYMENT_TARGET = 8.0;
@@ -521,6 +522,7 @@
 				DYLIB_CURRENT_VERSION = 1;
 				DYLIB_INSTALL_NAME_BASE = "@rpath";
 				FLUTTER_APPLICATION_PATH = path/to/application;
+				FLUTTER_PATH = path/to/flutter/repo;
 				INFOPLIST_FILE = FlutterApplication/Info.plist;
 				INSTALL_PATH = "$(LOCAL_LIBRARY_DIR)/Frameworks";
 				IPHONEOS_DEPLOYMENT_TARGET = 8.0;