Implement lastModified method on CodeStorage (to be able to understand when code needs to be regenerated)

[aormerod-gs]

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Test Results

   537 files     537 suites   22m 12s :stopwatch: 3 937 tests 3 905 :heavy_check_mark: 32 :zzz: 0 :x: 5 254 runs  5 222 :heavy_check_mark: 32 :zzz: 0 :x:

Results for commit d1582e82.