.addWatchFiles() dependencies can fail to trigger rebuilds in 1.21.0+

[tivac]

• Rollup Version: 1.21.0 and higher
• Operating System (or Browser): Windows 10
• Node Version: 12.8.0

How Do We Reproduce?

https://gist.github.com/tivac/ca2ff933d6dac4e06e8264800b0a1f98

Expected Behavior

Dependencies expressed using this.addWatchFile() would trigger invalidation of the calling module whenever the dependency changes.

Actual Behavior

The calling module is not invalidated.

Bug explanation

This bug was introduced by #3081 (which is an awesome change and a huge QoL improvement), but just cost me an evening chasing down a subtle issue.

Here's the specific problematic change.

invalidate(id: string, isTransformDependency: boolean) {
    this.invalidated = true;
    if (isTransformDependency) {
-        (this.cache.modules as ModuleJSON[]).forEach(module => {
-            if (!module.transformDependencies || module.transformDependencies.indexOf(id) === -1)
-                return;
+        for (const module of this.cache.modules) {
+            if (module.transformDependencies.indexOf(id) === -1) return;
            // effective invalidation
            module.originalCode = null as any;
-        });
+        }
    }
    this.watcher.invalidate(id);
}

When this code was refactored from this.cache.modules.forEach(..) to for(const module of this.cache.modules) { ... }, the return wasn't changed to a continue, so as soon as the first module that doesn't have a matching id in transformDependencies is found it halts iteration.