v0.6.0-nullsafety.1

• New asXyzOrThrow() methods as shorthand for .required().asXyz() featuring better error messages
  • asBoolOrThrow()
  • asDateTimeOrThrow()
  • asDoubleOrThrow()
  • asIntOrThrow()
  • letOrThrow()
  • asListOrThrow()
  • asMapOrThrow()
  • asStringOrThrow()
• New Pick.isAbsent getter to check if a value is absent or null #24. Absent could mean
  1. Accessing a key which doesn't exist in a Map
  2. Reading the value from List when the index is greater than the length
  3. Trying to access a key in a Map but the found data a Object which isn't a Map
• The String "true" and "false" are now parsed as boolean
• More nnbd refactoring