Make the observatory building be dependent on the pkg_packages target

With the current setup (as of revision 42679), a clean build of:
tools/build.py -mrelease build_observatory

Will throw an error since the args packages has not yet been symlinked into the out/ReleaseIA32/packages location.

R=ager@google.com

Review URL: https://codereview.chromium.org//838173002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@42683 260f80e4-7a28-3924-810f-c04153c831b5

runtime/observatory/observatory.gypi

@@ -42,6 +42,8 @@
       'dependencies': [
         'dart_bootstrap#host',
         'fetch_observatory_deps#host',
+        # We use packages for building
+        '../pkg/pkg.gyp:pkg_packages#target',
       ],
       'toolsets': ['host'],
       'sources': [
@@ -175,6 +177,7 @@
           'action_name': 'pub_build_observatory',
           'inputs': [
             '../../tools/observatory_tool.py',
+            '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
             '<@(_sources)',
           ],
           'outputs': [
@@ -222,4 +225,4 @@
       ],
     },
   ],
-}
+}