Valid ES5 code breaks after auto-update to ES2015 target

[elvisbegovic]

🐞 Bug report

at runtime, with and without AOT enabled

Is this a regression?

Yes, before auto-update target to es2015 it was valid es5 code. I think because in es5 when creating new variable without using (var,let,const), the variable is automatically global :

🔬 Minimal Reproduction

Anywhere in your angular app with target:es2015 write:

let t = 'b';

switch (t) {
  case 'a':
    let numberMin = 1;
    console.log(numberMin)
    break;

  case 'b':
    numberMin = 2;
    console.log(numberMin)
    break;
}

🔥 Exception or Error

ERROR ReferenceError: numberMin is not defined

🌍 Your Environment

Angular CLI: 8.0.0-beta.16
Node: 10.10.0
OS: win32 x64
Angular: 8.0.0-beta.13
... animations, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.800.0-beta.16
@angular-devkit/build-angular     0.800.0-beta.16
@angular-devkit/build-optimizer   0.800.0-beta.16
@angular-devkit/build-webpack     0.800.0-beta.16
@angular-devkit/core              8.0.0-beta.16
@angular-devkit/schematics        8.0.0-beta.16
@angular/cli                      8.0.0-beta.16
@ngtools/webpack                  8.0.0-beta.16
@schematics/angular               8.0.0-beta.16
@schematics/update                0.800.0-beta.16
rxjs                              6.4.0
typescript                        3.4.4
webpack                           4.30.0

Anything else relevant?

cc @filipesilva

This kind of code is easy to fix to be es2015 valid. But due to the fact it happens only "at runtime" it is dificult on existing angular project to see where&when it happens.

[filipesilva]

This is a problem with TS not identifying that the code should error. It is being tracked in microsoft/TypeScript#31167 and microsoft/TypeScript#19503.

[alexeagle]

We'll inform users about this in the 8.0 blog post - nothing we can do to fix this on Angular's side