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

runtime/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 import("runtime_args.gni")
 
 declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
   # Debug build of Dart so that clients can still use a Release build of Dart
   # while themselves doing a Debug build.
@@ skipping 117 matching lines @@
   } else {
     defines += [ "NDEBUG" ]
   }
 
   include_dirs = []
   if (dart_use_tcmalloc) {
     defines += [ "DART_USE_TCMALLOC" ]
     include_dirs += [ "../third_party/tcmalloc/gperftools/src" ]
   }
 
-  if (defined(is_fuchsia) && is_fuchsia) {
+  if (is_fuchsia) {
     defines += [ "DART_USE_JEMALLOC" ]
     include_dirs += [ "//magenta/third_party/ulib/jemalloc/include" ]
   }
 
   if (!is_win) {
     cflags = [
       "-Werror",
       "-Wall",
       "-Wextra",  # Also known as -W.
       "-Wno-unused-parameter",
@@ skipping 141 matching lines @@
     "third_party/double-conversion/src:libdouble_conversion",
     "vm:libdart_lib_jit",
     "vm:libdart_lib_nosnapshot_with_precompiler",
     "vm:libdart_vm_jit",
     "vm:libdart_vm_nosnapshot_with_precompiler",
   ]
   sources = [
     "vm/libdart_dependency_helper.cc",
   ]
 }