fix($rootScope): handle cyclic references in scopes when creating error messages

Use the new private function `stringify` to convert scope values to strings,
since this can cope with cyclic references and other oddities.

Closes #10085

Author: petebacondarwin

src/ng/rootScope.js

@@ -773,11 +773,11 @@ function $RootScopeProvider() {
                       if (ttl < 5) {
                         logIdx = 4 - ttl;
                         if (!watchLog[logIdx]) watchLog[logIdx] = [];
-                        logMsg = (isFunction(watch.exp))
-                            ? 'fn: ' + (watch.exp.name || watch.exp.toString())
-                            : watch.exp;
-                        logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
-                        watchLog[logIdx].push(logMsg);
+                        watchLog[logIdx].push({
+                          msg: isFunction(watch.exp) ? 'fn: ' + (watch.exp.name || watch.exp.toString()) : watch.exp,
+                          newVal: value,
+                          oldVal: last
+                        });
                       }
                     } else if (watch === lastDirtyWatch) {
                       // If the most recently dirty watcher is now clean, short circuit since the remaining watchers
@@ -810,7 +810,7 @@ function $RootScopeProvider() {
             throw $rootScopeMinErr('infdig',
                 '{0} $digest() iterations reached. Aborting!\n' +
                 'Watchers fired in the last 5 iterations: {1}',
-                TTL, toJson(watchLog));
+                TTL, watchLog);
           }
 
         } while (dirty || asyncQueue.length);

test/ng/rootScopeSpec.js

@@ -298,11 +298,11 @@ describe('Scope', function() {
           $rootScope.$digest();
         }).toThrowMinErr('$rootScope', 'infdig', '100 $digest() iterations reached. Aborting!\n' +
             'Watchers fired in the last 5 iterations: ' +
-            '[["a; newVal: 96; oldVal: 95","b; newVal: 97; oldVal: 96"],' +
-            '["a; newVal: 97; oldVal: 96","b; newVal: 98; oldVal: 97"],' +
-            '["a; newVal: 98; oldVal: 97","b; newVal: 99; oldVal: 98"],' +
-            '["a; newVal: 99; oldVal: 98","b; newVal: 100; oldVal: 99"],' +
-            '["a; newVal: 100; oldVal: 99","b; newVal: 101; oldVal: 100"]]');
+            '[[{"msg":"a","newVal":96,"oldVal":95},{"msg":"b","newVal":97,"oldVal":96}],' +
+            '[{"msg":"a","newVal":97,"oldVal":96},{"msg":"b","newVal":98,"oldVal":97}],' +
+            '[{"msg":"a","newVal":98,"oldVal":97},{"msg":"b","newVal":99,"oldVal":98}],' +
+            '[{"msg":"a","newVal":99,"oldVal":98},{"msg":"b","newVal":100,"oldVal":99}],' +
+            '[{"msg":"a","newVal":100,"oldVal":99},{"msg":"b","newVal":101,"oldVal":100}]]');
 
         expect($rootScope.$$phase).toBeNull();
       });