update build mac section in readme

engine/README.md

 
 ## How to Build (Mac)
 
-### Install Depot_tools
-```
-git clone 'https://chromium.googlesource.com/chromium/tools/depot_tools.git'
-
-export PATH="${PWD}/depot_tools:${PATH}"
-```
-
-### Build Skia
-```
-git clone https://skia.googlesource.com/skia.git
-
-git checkout chrome/m85
-
-bin/gn gen out/Debug
-
-python tools/git-sync-deps
-
-ninja -C out/Debug -k 0
-```
-
-Please ensure that you are using python2 when executing "python tools/git-sync-deps".
-
-
-### Build Flutter Engine
+### Build Dependencies
 
 Setting up the Engine development environment
 
 
 ```
+cd $FLUTTER_ROOT/flutter
-Apply changes to BUILD.gn (src/flutter/fml/BUILD.gn)
-
+Apply following to end of `flutter/third_party/txt/BUILD.gn`
-diff --git a/fml/BUILD.gn b/fml/BUILD.gn
-index 9b5626e78..da1322ce5 100644
---- a/fml/BUILD.gn
-+++ b/fml/BUILD.gn
-@@ -295,3 +295,10 @@ executable("fml_benchmarks") {
-     "//flutter/runtime:libdart",
+diff --git a/third_party/txt/BUILD.gn b/third_party/txt/BUILD.gn
+index 56b73a020..d42e88045 100644
+--- a/third_party/txt/BUILD.gn
++++ b/third_party/txt/BUILD.gn
+@@ -141,6 +141,7 @@ source_set("txt") {
+     "//third_party/harfbuzz",
+     "//third_party/icu",
+     "//third_party/skia",
++    "//third_party/skia/modules/skottie",
+ 
+   deps = [
+@@ -339,3 +340,10 @@ executable("txt_benchmarks") {
+     deps += [ "//third_party/skia/modules/skparagraph" ]
+   }
-+static_library("fml_lib") {
++static_library("txt_lib") {
-+    "//flutter/fml",
++    ":txt",
 +  ]
 +}
 ```
-cd engine/src
-
+cd $FLUTTER_ROOT
-
-ninja -C ./out/host_debug_unopt/ flutter/fml:fml_lib
+ninja -C out/android_debug_unopt/ flutter/third_party/txt:txt_lib
-### Build Lib
+### Prepare icu data (TODO: try to build it into the library directly!)
-set SKIA_ROOT and FLUTTER_ROOT to your $PATH. SKIA_ROOT is the root folder of your skia repository. FLUTTER_ROOT is the root folder of your flutter engine repository. 
+move the built icu data file from $FLUTTER_ROOT/out/host_debug_unopt/icudtl.dat to the StreamingAssets path
+### Creat symbolic
 
 Create symbolic as follows. Flutter engine txt include skia header in this pattern 'third_party/skia/*', so without symbolic, the txt lib will include skia
 header file in flutter engine, instead of headers in skia repo.