Suggestion: Let indexable types have return type (<type> | undefined)

[jpanda109]

TypeScript Version: 2.1.4

Allowing indexing of indexable interfaces act similar to ES6 Map.get seems like it would help the type safety of the language.

Example:

interface Foo {
    [name: string]: number
}
let foo: Foo = {};
let baz = foo["bar"];
// baz is of type number and compiles fine, even though is actually undefined
// possibly make baz of type (number | undefined) instead

Expected Behavior:
baz is of type (number | undefined)

Actual Behavior:
baz is of type number, despite actually being type undefined

[aluanhaddad]

Duplicate of #9235