[es6] stage Object.assign() / --harmony-object

BUG=v8:4007
LOG=N
R=

Review URL: https://codereview.chromium.org/1128873006

Cr-Commit-Position: refs/heads/master@{#28349}

src/flag-definitions.h

@@ -193,12 +193,12 @@ DEFINE_IMPLICATION(es_staging, harmony)
   V(harmony_unicode_regexps, "harmony unicode regexps")         \
   V(harmony_reflect, "harmony Reflect API")                     \
   V(harmony_destructuring, "harmony destructuring")             \
-  V(harmony_object, "harmony Object methods")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                               \
   V(harmony_rest_parameters, "harmony rest parameters") \
   V(harmony_spreadcalls, "harmony spread-calls")        \
+  V(harmony_object, "harmony Object methods")
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                                \