[fuchsia] GN build support for Fuchsia OS and fuchsia_test target

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.
 
 # TODO(zra): These build arguments should likely be moved to a gni file that is
 # included in BUILD.gn files that care about the values of the flags. For now,
 # since the GN build only happens as part of a Mojo build there is no need for
 # the indirection.
 declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
@@ skipping 138 matching lines @@
   if (dart_debug) {
     cflags += [
       "-O1",
     ]
   } else {
     cflags += [
       "-O3",
     ]
   }
 
-  if (is_asan) {
+  if (defined(is_asan) && is_asan) {
     ldflags = [
       "-Wl,-u_sanitizer_options_link_helper",
       "-fsanitize=address",
     ]
   }
 }
 
 static_library("libdart") {
   configs += [":dart_config",
               ":dart_product_config",
@@ skipping 53 matching lines @@
     "vm:libdart_lib_nosnapshot",
     "vm:libdart_lib",
     "vm:libdart_vm",
     "vm:libdart_platform",
     "third_party/double-conversion/src:libdouble_conversion",
   ]
   sources = [
     "vm/libdart_dependency_helper.cc",
   ]
 }