Fixed null reference exception in EcmaProvider.PopulateTree

EcmaProvider has the issue with fields and methods with "op_" prefix. It assumes that all fields and method with "op_" prefix are operators and breaks with null reference exception when trying to parse Xml generated from "static int op_Addition;" field or "static int op_SomebodyElse()" method.
I changed the behaviour of determining the Operator type from ecma xml. Now operators are only methods with "op_" prefix and is marked with "specialname" keyword in IL signature.
  Also added a test case.