Add --packages option to gen_snapshot in order to accept package map files (see https://github.com/…

runtime/tools/create_snapshot_bin.py

Index: runtime/tools/create_snapshot_bin.py
diff --git a/runtime/tools/create_snapshot_bin.py b/runtime/tools/create_snapshot_bin.py
index 78213cf304c600ecaa6c83ee1fc134b7a22f90a8..65e56ded00c4a23e8c24d8a135d7bc58c4df4c89 100755
--- a/runtime/tools/create_snapshot_bin.py
+++ b/runtime/tools/create_snapshot_bin.py
@@ -45,6 +45,9 @@ def BuildOptions():
   result.add_option("--package_root",
       action="store", type="string",
       help="path used to resolve package: imports.")
+  result.add_option("--packages",
+      action="store", type="string",
+      help="package config file used to reasolve package: imports.")
   result.add_option("--url_mapping",
       default=[],
       action="append",
@@ -112,6 +115,10 @@ def Main():
   if options.package_root:
     script_args.append(''.join([ "--package_root=", options.package_root]))
 
+  # Pass along the packages if there is one.
+  if options.packages:
+    script_args.append(''.join([ "--packages=", options.packages]))
+
   # First setup the vm isolate and regular isolate snapshot output filename.
   script_args.append(''.join([ "--vm_isolate_snapshot=",
                                options.vm_output_bin ]))