Add a test verifying that |let<newline>let| isn't subjected to ASI, matches LexicalDeclaration, and is then treated as an early error

Add tests verifying that ASI can't occur after the leading let/const in |let let|, |let let = "...";|, and |const let = "...";|.  Each matches the LexicalDeclaration production, then triggers an early error before evaluation because it attempts to bind "let".

Add test cases for recent ES2016 fixes

- lastIndex in RegExp.prototype[Symbol.split]: tc39/ecma262@08b4756747f9c967058239df627da544412740a6
- Missing number conversion in DataView.prototype.setXXX: tc39/ecma262@4f875fe96dcbd5e6363dd3d5b510cd5109199f5b
- Negative zero byteoffset in TypedArray: tc39/ecma262@2d1ed20db778e8f69f66ba76b351b9322fd38495
- EvalDeclarationInstantiation throws TypeError: tc39/ecma262@2be6968715946a4763f6ca8633e311ab7ce63577
- BindingRestElement allows BindingPattern: tc39/ecma262@d322357e6be95bc4bd3e03f5944a736aac55fa50
- Eval in parameters with computed property keys: tc39/ecma262@04e2e9b7197a33612202e85065e3f8d8385fbcef
- Use strict with non-simple parameters: tc39/ecma262@15b0db41edd0e519e94668bf13765fe458840766
- __proto__ in strict mode: tc39/ecma262@5c1984334d6d42538527bd022e68bfbfee5cb652

Add tests for ArrayBuffer and ArrayBuffer.prototype.slice

Improve test coverage for various Promise methods

Remove unnecessary ES2015 syntax; add feature flags where appropriate.

This allows these tests to be more easily reused to test Promise
implementations in isolation.

Updates for ES2016 Draft 2015-12-01

Test built-in function requirements of Promise and Proxy helper functions

- RegExp.prototype[Symbol.split] calls ToUint32 (https://github.com/tc39/ecma262/issues/92)
- Species lookup removed from Promise.all and Promise.race (https://github.com/tc39/ecma262/issues/151)
- Generator functions are no longer constructors (https://github.com/tc39/ecma262/pull/171)

Fixes #444

Revert primitive wrapper prototypes and String#split to ES5

- if-statement completion value semantics was not properly updated in ES2015

- Don't interpret date-only as local time

- Revert Number, Boolean and String prototypes to ES5 semantics
- Revert String.prototype.split's to ES5 behavior