Use before declaration errors when using let statements within a switch case

TypeScript Version: 2.5.3

Code

function test(value) {
    switch (value) {
        case 1:
            let foo = 10;
            break;
        case 2:
            foo = 20; // Runtime ReferenceError: foo is not defined
            break;
        default:
    }
}

test(2);

tsc --target es2015 (or higher)

Expected behavior:
This should probably produce a compile time error: Block-scoped variable 'pen' used before its declaration. on reference to foo in case 2.

Actual behavior:

The code type-checks and compiles fine, however, when running the es2015 transpiled output, you will receive a ReferenceError error at runtime. Ideally this type of error could be caught at compile time.

Start with the provided switch-case example using TypeScript 2.5.3 and compile it with --target es2015 or higher. Trace the type-checking path for block-scoped let declarations and references across case clauses. Done means the reference to foo in case 2 produces a compile-time block-scoped-variable-before-declaration diagnostic instead of compiling to a runtime ReferenceError.