Prevent mock clock timing fns from being spied on

Fixes #826

src/core/Clock.js

@@ -5,6 +5,8 @@ getJasmineRequireObj().Clock = function() {
     process.versions &&
     typeof process.versions.node === 'string';
 
+  const IsMockClockTimingFn = Symbol('IsMockClockTimingFn');
+
   /**
    * @class Clock
    * @since 1.3.0
@@ -183,6 +185,10 @@ callbacks to execute _before_ running the next one.
       advanceUntilModeChanges();
     };
 
+    setTimeout[IsMockClockTimingFn] = true;
+    clearTimeout[IsMockClockTimingFn] = true;
+    setInterval[IsMockClockTimingFn] = true;
+    clearInterval[IsMockClockTimingFn] = true;
     return this;
 
     // Advances the Clock's time until the mode changes.
@@ -335,5 +341,6 @@ callbacks to execute _before_ running the next one.
     return this;
   };
 
+  Clock.IsMockClockTimingFn = IsMockClockTimingFn;
   return Clock;
 };

src/core/SpyRegistry.js

@@ -41,6 +41,12 @@ getJasmineRequireObj().SpyRegistry = function(j$) {
         throw new Error(getErrorMsg(methodName + '() method does not exist'));
       }
 
+      // Spying on mock clock timing fns would prevent the real ones from being
+      // restored.
+      if (obj[methodName] && obj[methodName][j$.Clock.IsMockClockTimingFn]) {
+        throw new Error("Mock clock timing functions can't be spied on");
+      }
+
       if (obj[methodName] && j$.isSpy(obj[methodName])) {
         if (this.respy) {
           return obj[methodName];