Skip hashing for files that won't ever exist, don't create an empty a…

…rray just to discard it.

packages/broccoli-eyeglass/src/broccoli_sass_compiler.ts

@@ -690,32 +690,48 @@ export default class BroccoliSassCompiler extends BroccoliPlugin {
     return this.outputs[this.relativize(file)] || new Set();
   }
 
+  /**
+   * Some filenames returned from importers are not really files
+   * on disk. These three prefixes are used in eyeglass.
+   * Skipping a read on these files avoids a more expensive fs call
+   * and exception handling.
+   * @param filename a filename returned from an importer
+   */
+  isNotFile(filename: string): boolean {
+    return filename.startsWith("already-imported:") ||
+           filename.startsWith("autoGenerated:") ||
+           filename.startsWith("fs:");
+  }
+
   /* hash all dependencies synchronously and return the files that exist
    * as an array of pairs (filename, hash).
    */
   hashDependencies(details: CompilationDetails): Array<[string, string]> {
-    let depsWithHashes = new Array<[string, string] | []>();
+    let depsWithHashes = new Array<[string, string]>();
 
     this.dependenciesOf(details.fullSassFilename).forEach(f => {
       try {
-        let h = this.hashForFile(f);
+        if (this.isNotFile(f)) {
+          persistentCacheDebug("Ignoring non-file dependency: %s", f);
+        } else {
+          let h = this.hashForFile(f);
 
-        if (f.startsWith(details.srcPath)) {
-          f = f.substring(details.srcPath.length + 1);
+          if (f.startsWith(details.srcPath)) {
+            f = f.substring(details.srcPath.length + 1);
+          }
+          depsWithHashes.push([f, h.digest("hex")]);
         }
-        depsWithHashes.push([f, h.digest("hex")]);
       } catch (e) {
         if (typeof e === "object" && e !== null && e.code === "ENOENT") {
           persistentCacheDebug("Ignoring non-existent file: %s", f);
-          depsWithHashes.push([]);
         } else {
           throw e;
         }
       }
     });
 
     // prune out the dependencies that weren't files.
-    return depsWithHashes.filter(dwh => dwh.length > 0) as Array<[string, string]>;
+    return depsWithHashes;
   }
 
   /* read all output files asynchronously and return the contents