Update vm/BUILD.gn to account for stricter GN warnings.

runtime/vm/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.
 
 config("libdart_vm_config") {
   # TODO(zra, jamesr): This check can go away after some problems with the
   # fuchsia toolchain definition are fixed.
   if (!defined(is_fuchsia) || !is_fuchsia) {
     libs = [ "dl" ]
 
@@ skipping 184 matching lines @@
     if (do_patch) {
       patch_path = lib[4]
       generate_library_source("generate_${filename}_patch_cc_file") {
         libname = libname
         filename = filename
         kind = "patch"
         path = patch_path
         output = "$target_gen_dir/${filename}_patch_gen.cc"
       }
     }
+    lib_sources_gypi = {}
     lib_sources_gypi =
         exec_script("../../tools/gypi_to_gn.py",
                     [rebase_path("../lib/${filename}_sources.gypi")],
                     "scope",
                     ["../lib/${filename}_sources.gypi"])
     libsources += rebase_path(lib_sources_gypi.sources, ".", "../lib")
     liboutputs += ["$target_gen_dir/${filename}_gen.cc"]
     libdeps += [":generate_${filename}_cc_file"]
     if (do_patch) {
       liboutputs += ["$target_gen_dir/${filename}_patch_gen.cc"]
@@ skipping 61 matching lines @@
     ["developer", "developer", true, "../../sdk/lib/developer", "../lib"],
     ["_internal", "internal", true, "../../sdk/lib/internal", "../lib"],
     ["isolate", "isolate", true, "../../sdk/lib/isolate", "../lib"],
     ["math", "math", true, "../../sdk/lib/math", "../lib"],
     ["mirrors", "mirrors", true, "../../sdk/lib/mirrors", "../lib"],
     ["profiler", "profiler", false, "../../sdk/lib/profiler"],
     ["typed_data", "typed_data", false, "../lib"],
     ["_vmservice", "vmservice", true, "../../sdk/lib/vmservice", "../lib"],
   ]
 }