Missing imports in ESM `.js` files do not produce errors (only warnings)

[gkalpak]

🐞 Bug report

Command (mark with an x)

- [x] build

Is this a regression?

N/A

Description

When an esm5/esm2015 .js file is trying to import a non-existing export, there is a warning emitted during ng build, but no hard error. Note this only affects .js files, since missing imports fail in .ts files.

For context:
We ran into this in angular/angular#30362, where esm files in @angular/material ended up trying to import missing exports from @angular/core. The build succeeded (with warnings only), but the app is broken.
(See angular/angular#30362 (comment) for more details.)

🔬 Minimal Reproduction

This is a contrived minimal reproduction that produces the same error as the one seen in angular/angular#30362:

• Download and unzip test-app.zip.
• Run npm install.
• Run npm run build.

The repro project was created with ng new test-app --defaults and the following changes:

• Change src/main.ts to src/main.js in angular.json.
• Add src/main.js in tsconfig.json > include.
• Rename src/main.ts to src/main.js and update its content to import missingExport from @angular/core.

🔥 Exception or Error

WARNING in ./src/main.js 2:12-25
"export 'missingExport' was not found in '@angular/core'

🌍 Your Environment

Angular CLI: 8.0.0-rc.3
Node: 10.15.3
OS: win32 x64
Angular: 8.0.0-rc.3
... animations, cli, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.800.0-rc.3
@angular-devkit/build-angular     0.800.0-rc.3
@angular-devkit/build-optimizer   0.800.0-rc.3
@angular-devkit/build-webpack     0.800.0-rc.3
@angular-devkit/core              8.0.0-rc.3
@angular-devkit/schematics        8.0.0-rc.3
@ngtools/webpack                  8.0.0-rc.3
@schematics/angular               8.0.0-rc.3
@schematics/update                0.800.0-rc.3
rxjs                              6.4.0
typescript                        3.4.5
webpack                           4.30.0