Support for new features of TypeScript.

[shiwano]

• Namespace
• Decorator
  • Decorators microsoft/TypeScript#2249
• Generic type alias.
  • Type aliases requiring type parameters microsoft/TypeScript#1616
• Generators
  • Proposal for generators design microsoft/TypeScript#2873
• Intersection types
  • Intersection types microsoft/TypeScript#3622
• Abstract classes and methods
  • Proposal for Abstract Classes and Methods microsoft/TypeScript#3578
• User-defined type guard functions
  • Support user-defined type guard functions microsoft/TypeScript#1007
• ES6 Symbols
• tsconfig.json
• const, let variables
• built-in symbol methods