[tools][gen] Fix nil-deref on non-existent include

File() used to get-or-create. Now you need to call AddFile() first.

Change-Id: I5166258abe80f192bfec48e823d94459668af01b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/146900
Commit-Queue: Antonio Maiorano <[email protected]>
Auto-Submit: Ben Clayton <[email protected]>
Reviewed-by: Antonio Maiorano <[email protected]>
Kokoro: Kokoro <[email protected]>

Author: ben-clayton

tools/src/cmd/gen/build/build.go

@@ -233,10 +233,10 @@ func scanSourceFiles(p *Project) error {
 					if strings.HasPrefix(include.path, srcTint) {
 						// #include "src/tint/..."
 						include.path = include.path[len(srcTint)+1:] // Strip 'src/tint/'
-						includeFile := p.File(include.path)
 
-						if _, err := os.Stat(includeFile.AbsPath()); err != nil {
-							return fmt.Errorf(`%v:%v includes non-existent file '%v'`, file.Path(), include.line, includeFile.AbsPath())
+						includeFile := p.File(include.path)
+						if includeFile == nil {
+							return fmt.Errorf(`%v:%v includes non-existent file '%v'`, file.Path(), include.line, include.path)
 						}
 
 						dependencyKind := targetKindFromFilename(include.path)