chore(testing): Refactor test methods to have a uniform interface.

Remove FunctionWithParamTokens.

All test wrappers async, fakeAsync and inject now return just a Function instead of FunctionWithParamTokens. This makes them directly consumable by the test framework. Also the test framework code does not have to handle a union of Function and FunctionWithParamTokens everywhere.

The Function returned by the above methods are considered asynchronous by the test framework if they return a Promise, synchronous otherwise.

Closes #8257

modules_dart/angular2_testing/lib/angular2_testing.dart

@@ -79,8 +79,7 @@ dynamic _runInjectableFunction(Function fn) {
     tokens.add(token);
   }
 
-  var injectFn = new FunctionWithParamTokens(tokens, fn, false);
-  return _testInjector.execute(injectFn);
+  return _testInjector.execute(tokens, fn);
 }
 
 /// Use the test injector to get bindings and run a function.