Don't clobber previous custom plans with later plans

[finishes #37288941]

src/core/SpyFactory.js

@@ -6,18 +6,18 @@ getJasmineRequireObj().SpyFactory = function(j$) {
     this.createSpy = function(name, originalFn) {
       return j$.Spy(name, originalFn, getCustomStrategies());
     };
-  
+
     this.createSpyObj = function(baseName, methodNames) {
       var baseNameIsCollection = j$.isObject_(baseName) || j$.isArray_(baseName);
-  
+
       if (baseNameIsCollection && j$.util.isUndefined(methodNames)) {
         methodNames = baseName;
         baseName = 'unknown';
       }
-   
+
       var obj = {};
       var spiesWereSet = false;
-   
+
       if (j$.isArray_(methodNames)) {
         for (var i = 0; i < methodNames.length; i++) {
           obj[methodNames[i]] = self.createSpy(baseName + '.' + methodNames[i]);
@@ -32,11 +32,11 @@ getJasmineRequireObj().SpyFactory = function(j$) {
           }
         }
       }
-   
+
       if (!spiesWereSet) {
         throw 'createSpyObj requires a non-empty array or object of method names to create spies for';
       }
-   
+
       return obj;
     };
   }

src/core/SpyStrategy.js

@@ -20,20 +20,24 @@ getJasmineRequireObj().SpyStrategy = function(j$) {
     var k, cs = options.customStrategies || {};
     for (k in cs) {
       if (j$.util.has(cs, k) && !this[k]) {
-        this[k] = function() {
-          var plan = cs[k].apply(null, arguments);
-
-          if (!j$.isFunction_(plan)) {
-            throw new Error('Spy strategy must return a function');
-          }
-
-          this.plan = plan;
-          return this.getSpy();
-        };
+        this[k] = createCustomPlan(cs[k]);
       }
     }
   }
 
+  function createCustomPlan(factory) {
+    return function() {
+      var plan = factory.apply(null, arguments);
+
+      if (!j$.isFunction_(plan)) {
+        throw new Error('Spy strategy must return a function');
+      }
+
+      this.plan = plan;
+      return this.getSpy();
+    };
+  }
+
   /**
    * Execute the current spy strategy.
    * @name SpyStrategy#exec

src/core/requireInterface.js

@@ -280,5 +280,18 @@ getJasmineRequireObj().interface = function(jasmine, env) {
     return env.createSpyObj(baseName, methodNames);
   };
 
+  /**
+   * Add a custom spy strategy for the current scope of specs.
+   *
+   * _Note:_ This is only callable from within a {@link beforeEach}, {@link it}, or {@link beforeAll}.
+   * @name jasmine.addSpyStrategy
+   * @function
+   * @param {String} name - The name of the strategy (i.e. what you call from `and`)
+   * @param {Function} factory - Factory function that returns the plan to be executed.
+   */
+  jasmine.addSpyStrategy = function(name, factory) {
+    return env.addSpyStrategy(identifier, factory);
+  };
+
   return jasmineInterface;
 };