add support for ES6

[michaelficarra]

Grammar:

• block-scoped variable declarations (let/const)
• block-level function declarations
• spread arguments (ES6: spread in call/new arguments and array initialiser #47; started for array initialisers in ES6: restrict assignment positions to bindings #30)
• rest parameters (rolled into ES6: restrict assignment positions to bindings #30)
• import and export declarations (ES6: Imports and Exports #75)
• destructuring (ES6: restrict assignment positions to bindings #30)
• classes (ES6 Support: Classes #82)
• arrow functions (ES6: arrow functions #24)
• generator functions + yield (ES6: Generator functions, Generator Methods and Yield. #71)
• default function parameters (rolled into ES6: restrict assignment positions to bindings #30)
• method definitions (in object literal) (ES6: add method definition support #40)
• binary/octal integer literals (ES6: add binary and octal integer literal support #16)
• noctals (ES6: Noctal support #117)
• shorthand object property notation (ES6: add shorthand object property notation support #37)
• remove duplicate object property (besides __proto__) strict mode restriction (ES6: remove duplicate object property (besides __proto__) restriction #28)
• computed property names (ES6: computed property names #26)
• template strings (ES6: template strings #27)
• for-of (ES6: for-of #29)
• restrict assignment positions to bindings (assignment LHS, catch param, for-in left, etc.) (ES6: restrict assignment positions to bindings #30)
• allow missing semicolon after do-while
• super calls and super member accesses (ES6: SuperCalls and SuperProperty #100)
• new.target (ES6: new.target #101)
• extended unicode escapes in strings, templates, and regexps "\u{123456}" (ES6: extended unicode escapes in strings, templates, and regexps "\u{123456}" #78)
• validate surrogate pairs in strings, templates, and regexps(deferred)
• allow HTML-style comments in Script (not Module) code (ES6: allow HTML-style comments in Script (not Module) code #109)
• disallow initialiser in for-in and for-of head (ES6: disallow initialiser in for-in and for-of head #68)
• finish shift-spec ES6 milestone
• add parseScript and parseModule interfaces (ES6: Add parseModule interface #98)

Verification:

• go through Annex A to confirm we cover the entire grammar
• go through Annex B, C, D, E to confirm we've not missed any incompatibilities
• confirm we have failure tests for each occurrence of "it is a syntax error if"

Create an issue (or comment on an existing one) if you want to start working on one.