fix(angular.copy): support circular references in the value being copied

Closes #7618

src/Angular.js

@@ -757,7 +757,7 @@ function isLeafNode (node) {
  </file>
  </example>
  */
-function copy(source, destination) {
+function copy(source, destination, stackSource, stackDest) {
   if (isWindow(source) || isScope(source)) {
     throw ngMinErr('cpws',
       "Can't copy! Making copies of Window or Scope instances is not supported.");
@@ -767,22 +767,40 @@ function copy(source, destination) {
     destination = source;
     if (source) {
       if (isArray(source)) {
-        destination = copy(source, []);
+        destination = copy(source, [], stackSource, stackDest);
       } else if (isDate(source)) {
         destination = new Date(source.getTime());
       } else if (isRegExp(source)) {
         destination = new RegExp(source.source);
       } else if (isObject(source)) {
-        destination = copy(source, {});
+        destination = copy(source, {}, stackSource, stackDest);
       }
     }
   } else {
     if (source === destination) throw ngMinErr('cpi',
       "Can't copy! Source and destination are identical.");
+
+    stackSource = stackSource || [];
+    stackDest = stackDest || [];
+
+    if (isObject(source)) {
+      var index = indexOf(stackSource, source);
+      if (index !== -1) return stackDest[index];
+
+      stackSource.push(source);
+      stackDest.push(destination);
+    }
+
+    var result;
     if (isArray(source)) {
       destination.length = 0;
       for ( var i = 0; i < source.length; i++) {
-        destination.push(copy(source[i]));
+        result = copy(source[i], null, stackSource, stackDest);
+        if (isObject(source[i])) {
+          stackSource.push(source[i]);
+          stackDest.push(result);
+        }
+        destination.push(result);
       }
     } else {
       var h = destination.$$hashKey;
@@ -790,10 +808,16 @@ function copy(source, destination) {
         delete destination[key];
       });
       for ( var key in source) {
-        destination[key] = copy(source[key]);
+        result = copy(source[key], null, stackSource, stackDest);
+        if (isObject(source[key])) {
+          stackSource.push(source[key]);
+          stackDest.push(result);
+        }
+        destination[key] = result;
       }
       setHashKey(destination,h);
     }
+
   }
   return destination;
 }

src/ng/parse.js

@@ -1072,7 +1072,7 @@ function $ParseProvider() {
               self.$$postDigestQueue.push(function () {
                 // create a copy if the value is defined and it is not a $sce value
                 if ((stable = isDefined(lastValue)) && !lastValue.$$unwrapTrustedValue) {
-                  lastValue = copy(lastValue);
+                  lastValue = copy(lastValue, null);
                 }
               });
             }

src/ng/rootScope.js

@@ -702,7 +702,7 @@ function $RootScopeProvider(){
                                && isNaN(value) && isNaN(last)))) {
                       dirty = true;
                       lastDirtyWatch = watch;
-                      watch.last = watch.eq ? copy(value) : value;
+                      watch.last = watch.eq ? copy(value, null) : value;
                       watch.fn(value, ((last === initWatchVal) ? value : last), current);
                       if (ttl < 5) {
                         logIdx = 4 - ttl;