[SalesForce] Best tool for seeing highlighted covered Apex Code while writing test class

Update: 10th May: Salesforce have confirmed this is an issue and appears to be resolved in Summer'13.

Confirmed Platform Bug Reproduced I spent a good portion of the weekend trying to get coverage for a WSDL2Apex class myself. Eventually whittled it down to a bug in the platform. The bug currently resides around tests that only invoke methods on "inner classes".

This is the standard pattern for classes generated by WSDL2Apex of course. The bug extends to the point that not only is code coverage not captured, it excludes the whole class from being required for code coverage, which explains the package upload behaviour. I reproduced it in the following...

public class TestA 
{
      public TestA() { }

      public class TestB 
      {
          public Integer doSomething()
          {
              Integer x = 1;
              Integer y = x;
              return y;         
          }
     }  
}

@IsTest
private with sharing class TestATest 
{
    private static testmethod void testMe()
    {
        // Uncomment the line below to work around the lack of code coverage bug
        // TestA testAObj = new TestA();

        TestA.TestB testBObj = new TestA.TestB();
        System.assertEquals(1, testBObj.doSomething());
    }
}

Workaround is to edit your generated class and put in a constructor and then invoke this from each of your test methods. If your concerned developers will attempt to use it (not that it would be much good, you can put an Test.isTesting() assert in it). Once we see a for fix this (I've raised case 09233301), this workaround can be left in or removed.

I'll update this answer when I get a response and/or the bug listed publicly. Note we have tests that do cover WSDL2Apex classes from the past, so this to me confirms this was at one point needed and is a regression. I did ponder if it was related to just WSDL2Apex generated classes only at first, which was starting to make sense, since they are none functional and system generated, shame...

Pattern for Generating 100% Coverage on WSDL2Apex Generated Classes. This is something I plan to blog about next weekend, but if your interested in taking a look at an approach I've used (irrespective of this issue) you can take a look at this. I'll update this answer when I do.

It sounds like you may have a specific problem related to mocking; however, for those experiencing the general problem stated in the question title, "Test classes are included with 0% in overall code coverage calculation", there is another cause for this issue. As @Bujji stated in his answer, it does occur (as of Spring 15) that changing a class from a non-test class to a test class can leave coverage data behind in the org that does not appear to be cleaned up by running all tests or by clearing all test data. I have just experience this issue; in a code base that I inherited, a test utility (factory) class was not marked @isTest, and so was generating coverage - incomplete coverage, in fact. I marked it @isTest, but the class didn't get removed from coverage reporting - it just dropped to 0%, lowering my overall coverage.

Instead of deleting and re-creating the class, you can clean up the old coverage data. The documentation for this known issue (resolved in Spring 15) describes a method for clearing old coverage data which resolves this issue:

1) [experience issue]

2) In the Developer console run this Tooling API query

SELECT Id, ApexClassOrTriggerId, ApexClassOrTrigger.Name,
       NumLinesCovered, NumLinesUncovered 
  FROM ApexCodeCoverageAggregate 

3) Select all results and hit delete

4) Run all tests

If you haven't seen it before, the Query tab of the Developer console has a little "use tooling api" checkbox you'll need to check.