CodeCoverage again

1) Removed CodeCoverageStringTextSource debug assertions/failures.
Why? 
Running SD5 in debug mode large number of assertions is revealed.
Why?
When using CodeContract ContractClass/ContractClassFor attributes,
ccrewrite inserts in "ContractClass" attributed class/interface SequencePoints from "ContractClassFor" attributed class. If "ContractClassFor" class is in another file, CodeCoverageStringTextSource.GetText(..) receives invalid line/column/endline/endcolumn arguments that fails or do not exists in file containing "ContractClass" attributed class/interface.
Why?
Afaik, cannot avoid invalid requests, because to filter them out, first need GetText() for all SequencePoints.

2) CodeCoverageMethodElement.cs changes & refactoring 
Why?
While trying to fix 1), in some rare cases SequencePoints are omitted.

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