Add ClosureInfo for closures.

test/hello_world/hello_world.js.info.json

 {
   "elements": {
     "library": {
       "0": {
         "id": "library/0",
         "kind": "library",
         "name": "<unnamed>",
         "size": 85,
         "children": [
           "function/0"
@@ skipping 1434 matching lines @@
         "inferredType": "[null|subclass=Object]",
         "code": null,
         "type": "List<K>"
       },
       "62": {
         "id": "field/62",
         "kind": "field",
         "name": "message",
         "size": 0,
         "outputUnit": "outputUnit/0",
-        "coverageId": "4090",
+        "coverageId": "4087",
         "parent": "class/143",
         "children": [],
         "inferredType": "Value mask: [\"Intercepted function with no arguments.\"] type: [exact=JSString]",
         "code": null,
         "type": "dynamic"
       },
       "63": {
         "id": "field/63",
         "kind": "field",
         "name": "message",
@@ skipping 329 matching lines @@
         "size": 0,
         "outputUnit": "outputUnit/0",
         "coverageId": "3883",
         "parent": "class/293",
         "children": [],
         "inferredType": "[null|subclass=Object]",
         "code": null,
         "type": "E"
       }
     },
-    "constant": {}
+    "constant": {},
+    "closure": {}
   },
   "holding": {
     "function/0": [
       {
         "id": "function/2",
         "mask": "null"
       },
       {
         "id": "function/1",
         "mask": "inlined"
@@ skipping 23 matching lines @@
     ],
     "function/1": [
       "field/12"
     ]
   },
   "outputUnits": [
     {
       "id": "outputUnit/0",
       "kind": "outputUnit",
       "name": null,
-      "size": 10124,
+      "size": 10116,
       "imports": [
         null
       ]
     }
   ],
-  "dump_version": 4,
+  "dump_version": 5,
   "deferredFiles": {},
   "dump_minor_version": "0",
   "program": {
     "entrypoint": "function/0",
-    "size": 10124,
+    "size": 10116,
     "dart2jsVersion": null,
-    "compilationMoment": "2016-09-30 16:14:58.038530",
-    "compilationDuration": 2726976,
+    "compilationMoment": "2016-10-06 10:14:59.145665",
+    "compilationDuration": 2729236,
     "toJsonDuration": 4000,
     "dumpInfoDuration": 0,
     "noSuchMethodEnabled": false,
     "minified": false
   }
 }