Better checking for null/undefined in operands of expressions

TypeScript 2.2 improves checking of nullable operands in expressions. Specifically, these are now flagged as errors:

• If either operand of a + operator is nullable, and neither operand is of type any or string.
• If either operand of a -, , *, /, %, <<, >>, >>>, &, |, or ^ operator is nullable.
• If either operand of a <, >, <=, >=, or in operator is nullable.
• If the right operand of an instanceof operator is nullable.
• If the operand of a +, -, ~, ++, or — unary operator is nullable.An operand is considered nullable if the type of the operand is null or undefined or a union type that includes null or undefined. Note that the union type case only only occurs in —strictNullChecks mode because null and undefined disappear from unions in classic type checking mode.