[infra] Assembles the SDK using GN rather than create_sdk.py

tools/gn.py

Index: tools/gn.py
diff --git a/tools/gn.py b/tools/gn.py
index 71823e0555a2624978a0d9c9373931a42d058116..f0c1d91460c0966593026e2e0ba24b16df52e6d1 100755
--- a/tools/gn.py
+++ b/tools/gn.py
@@ -25,6 +25,8 @@ DART_USE_TSAN = "DART_USE_TSAN"  # Use instead of --tsan
 DART_USE_WHEEZY = "DART_USE_WHEEZY"  # Use instread of --wheezy
 DART_USE_TOOLCHAIN = "DART_USE_TOOLCHAIN"  # Use instread of --toolchain-prefix
 DART_USE_SYSROOT = "DART_USE_SYSROOT"  # Use instead of --target-sysroot
+# use instead of --platform-sdk
+DART_MAKE_PLATFORM_SDK = "DART_MAKE_PLATFORM_SDK"
 
 def UseASAN():
   return DART_USE_ASAN in os.environ
@@ -54,6 +56,10 @@ def TargetSysroot(args):
   return os.environ.get(DART_USE_SYSROOT)
 
 
+def MakePlatformSDK():
+  return DART_MAKE_PLATFORM_SDK in os.environ
+
+
 def GetOutDir(mode, arch, target_os):
   return utils.GetBuildRoot(HOST_OS, mode, arch, target_os)
 
@@ -197,6 +203,8 @@ 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
+
   # Setup the user-defined sysroot.
   if gn_args['target_os'] == 'linux' and args.wheezy and not crossbuild:
     gn_args['dart_use_wheezy_sysroot'] = True
@@ -357,6 +365,10 @@ def parse_args(args):
       help='Disable MSAN',
       dest='msan',
       action='store_false')
+  other_group.add_argument('--platform-sdk',
+      help='Directs the create_sdk target to create a smaller "Platform" SDK',
+      default=MakePlatformSDK(),
+      action='store_true')
   other_group.add_argument('--target-sysroot', '-s',
       type=str,
       help='Comma-separated list of arch=/path/to/sysroot mappings')