Changes to prevent coverage metrics from being reset each time an instrumented file is loaded.

Added a null check in the instrumentation JavaScript to prevent resetting coverage metrics each time the file is loaded. This was an issue for a project that uses Require.js contexts to isolate the modules for each Jasmine spec, which allows each spec to replace modules with stubs or mocks without impacting the other specs. Each time a context loaded an instrumented file, it reset the coverage metrics for that file.