Test cases for contextual typing of union type members
Fix the logic in getting the contextual signature of union type as per spec
Fixes #1054
Tests for contextual index signature of union type
Let S be the set of types in U that has a string index signature. If S is not empty, U has a string index signature of a union type of the types of the string index signatures from each type in S.
Let S be the set of types in U that has a numeric index signature. If S is not empty, U has a numeric index signature of a union type of the types of the numeric index signatures from each type in S.
Merge pull request #1058 from Microsoft/contextualUnionTypeConformance

Contextual union type conformance test cases and Fix for contextual union signature