grammar errors should be reported as syntactic diagnostics

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.3.0-dev.20181222 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** **Code** ```ts // @filename: test.js function test(...args,) {} // trailing comma not allowed class C { async static foo() {} // modifiers are in the wrong order } @decorator // invalid decorator location async var v = 1; // invalid modifier location let {...rest: rest} = {}; // rest property cannot have a property name ``` **Expected behavior:** All errors above are reported in JS files as they result in parse errors when executed. **Actual behavior:** The above file has no errors. They are not reported as syntax errors because TypeScript's parser is more permissive than the spec in many cases. Since grammar errors are only reported during type-checking they are ignored in JS files without `// @ts-check`. Other JS parsers, e.g. javascript VMs, emit a parse error for the code above. **Related Issues:** #6107 suggests moving grammar errors to the binding phase #6802