Allow generator functions to be passed to .and.callFake

Fixes #1848.

spec/helpers/generator.js

@@ -0,0 +1,22 @@
+(function(env) {
+  function getGeneratorFuncCtor() {
+    try {
+      eval('var func = function*() {}');
+    } catch (e) {
+      return null;
+    }
+
+    return Object.getPrototypeOf(func).constructor;
+  }
+
+  env.makeGeneratorFunction = function(text) {
+    var GeneratorFunction = getGeneratorFuncCtor();
+    return new GeneratorFunction(text || '');
+  };
+
+  env.requireGeneratorFunctions = function() {
+    if (!getGeneratorFuncCtor()) {
+      env.pending('Environment does not support generator functions');
+    }
+  };
+})(jasmine.getEnv());