Flutter and Fuchsia build stripped down [platform] version of dart sdk, so make it default(while standalone builds full dart sdk)

Both Flutter and Fuchsia expect stripped dart in standard out directory, so make that default too(while standalone build puts stripped into exe.stripped/).

BUG=
R=zra@google.com

Review-Url: https://codereview.chromium.org/2998503002 .

sdk/BUILD.gn

@@ -17,14 +17,9 @@ import("../build/copy_tree.gni")
 
 declare_args() {
   # Build a SDK with less stuff. It excludes dart2js, ddc, and web libraries.
-  dart_platform_sdk = false
-}
-
-if (is_fuchsia ||
-    is_fuchsia_host ||
-    current_cpu == "arm64" ||
-    current_cpu == "arm") {
   dart_platform_sdk = true
+  # Path to stripped dart binary relative to build output directory.
+  dart_stripped_binary = "dart"
 }
 
 # The directory layout of the SDK is as follows:
@@ -226,13 +221,9 @@ copy("copy_dart") {
     sources = [
       "$dart_out/dart.exe",
     ]
-  } else if (is_fuchsia || is_fuchsia_host) {
-    sources = [
-      "$dart_out/dart",
-    ]
   } else {
     sources = [
-      "$dart_out/exe.stripped/dart",
+      "$dart_out/$dart_stripped_binary"
     ]
   }
   if (is_win) {

tools/gn.py

@@ -223,7 +223,9 @@ def ToGnArgs(args, mode, arch, target_os):
   gn_args['is_msan'] = args.msan and gn_args['is_clang']
   gn_args['is_tsan'] = args.tsan and gn_args['is_clang']
 
-  gn_args['dart_platform_sdk'] = args.platform_sdk
+  if not args.platform_sdk and not gn_args['target_cpu'].startswith('arm'):
+    gn_args['dart_platform_sdk'] = args.platform_sdk
+  gn_args['dart_stripped_binary'] = 'exe.stripped/dart'
 
   # Setup the user-defined sysroot.
   if gn_args['target_os'] == 'linux' and args.wheezy and not crossbuild: