Compiler API: Expose More from typescript.d.ts

TypeScript Version: 2.9.0-dev.20180426

While Traversing the TypeScript AST of a source file, there have been a number of helpful things I've found by extending ts.* from typescript.d.ts and I would like it if these could be exposed (or not marked /* @internal */).

Search Terms:
typescript.d.ts
Node.locals
Symbol.parent
getContainingFunction
isAssignmentTarget

Code

declare namespace ts {
    interface Node {
        /* @internal */ readonly locals?: SymbolTable; // Locals associated with node (initialized by binding)
    }

    interface Symbol {
        /* @internal */ readonly parent?: Symbol; // Parent symbol
    }

    function getContainingFunction(node: Node): SignatureDeclaration | undefined;

    function isAssignmentTarget(node: Node): boolean;
}

Note:

• Added readonly for Node#locals & Symbol#parent
• For Symbol#parent, the goal is: given the symbol for a method (e.g., methodSymbol for Number#toString) find where that method is defined (e.g., Number)... maybe program.getTypeChecker().getSymbolAtLocation(methodSymbol.valueDeclaration.parent) is what is "supposed" to be used instead?
• For ts.getContainingFunction() it seems more appropriate to have a return type of SignatureDeclaration | undefined instead of SignatureDeclaration

Related Issues:
#15841

Start with the public declarations in typescript.d.ts and compare the listed Node.locals, Symbol.parent, getContainingFunction, and isAssignmentTarget members with their compiler implementations. Resolve the stated API and return-type questions, then verify that the intended members are exposed consistently for Compiler API consumers.