Silent (!) build failure when several source filepaths cannot be found

[vlukashov]

Description

When running polymer build in a project with a source file that imports several non-existing other source files, the build completes without errors but does not produce any output.

source file:

<link rel="import" href="../../bower_components/polymer/polymer-element.html">
<link rel="import" href="non-existent-file-1.html">
<link rel="import" href="non-existent-file-2.html">

output:

$ npm run build

> minimal@1.0.0 build /path/to/minimal-demo-case
> polymer build

info:    Clearing build/ directory...
$ ls -la bulid
total 8
drwxr-xr-x   3 viktor  staff   96 Dec  8 10:40 .
drwxr-xr-x  13 viktor  staff  416 Dec  8 10:40 ..
-rw-r--r--   1 viktor  staff  666 Dec  8 10:40 polymer.json
$

Versions & Environment

• Polymer CLI: 1.5.7
• node: 8.9.0
• Operating System: OS X 10.13.1

Steps to Reproduce

1. extract the attached project archive: minimal-demo-case.zip
2. install all project dependencies: run yarn install (or npm install)
3. run polymer build: yarn build (or npm run build)
4. verify that the build fails because the file non-existent-file-1.html does not exist
5. add a second include for a non-existent file to src/app/minimal-app.html: uncomment the <link rel="import" href="non-existent-file-2.html"> line
6. run polymer build: yarn build (or npm run build)
7. see that the build passes without errors, but the output folder (build/) is empty

Expected Results

The build should fail. There should be 2 errors in the console - 2 files are missing.

Actual Results

The build does not report any errors (as if it passed OK). The build output directory does not contain the build output.

[jsilvermist]

Seems like the same issue I observed in #904 but slightly more widespread than I had realized, I didn't realize normal imports would cause the same issue, not just lazy ones.