Add a --harmony-object-observe runtime flag (on by default)

test/mjsunit/es7/object-observe.js

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-// Flags: --harmony-proxies
+// Flags: --harmony-proxies --harmony-object-observe
 // Flags: --allow-natives-syntax
 
 var allObservers = [];
 function reset() {
   allObservers.forEach(function(observer) { observer.reset(); });
 }
 
 function stringifyNoThrow(arg) {
   try {
     return JSON.stringify(arg);
@@ skipping 1817 matching lines @@
   var ex;
   try {
     Object.observe({}, function(){}, "not an object");
   } catch (e) {
     ex = e;
   }
   assertInstanceof(ex, TypeError);
   assertEquals("Third argument to Object.observe must be an array of strings.",
                ex.message);
 })()