fix EEXIST error when rebuilding mirage sources

Author: dwickern

lib/incremental-typescript-compiler.js

@@ -50,15 +50,17 @@ module.exports = class IncrementalTypescriptCompiler {
     });
 
     let triggerTree = new Funnel(this._triggerDir, { destDir: 'app' });
-    let sourceDirectories = {
-      [`${this._relativeAppRoot()}/app`]: 'app',
-    };
 
-    this._maybeIncludeMirage(sourceDirectories);
+    let appTree = new TypescriptOutput(this, {
+      [`${this._relativeAppRoot()}/app`]: 'app',
+    });
 
-    let appTree = new TypescriptOutput(this, sourceDirectories);
+    let mirage = this._mirageDirectory();
+    let mirageTree = mirage && new TypescriptOutput(this, {
+      [mirage]: 'app/mirage',
+    });
 
-    let tree = new MergeTrees(addonAppTrees.concat([triggerTree, appTree]), { overwrite: true });
+    let tree = new MergeTrees(addonAppTrees.concat([triggerTree, appTree, mirageTree].filter(Boolean)), { overwrite: true });
     return new Funnel(tree, { srcDir: 'app' });
   }
 
@@ -162,7 +164,7 @@ module.exports = class IncrementalTypescriptCompiler {
     this._buildDeferred.resolve();
   }
 
-  _maybeIncludeMirage(sourceDirectories) {
+  _mirageDirectory() {
     let mirage = this.project.addons.find(addon => addon.name === 'ember-cli-mirage');
     if (mirage) {
       // Be a little defensive, since we're using an internal Mirage API
@@ -182,7 +184,7 @@ module.exports = class IncrementalTypescriptCompiler {
         if (source.indexOf(this.project.root) === 0) {
           source = source.substring(this.project.root.length + 1);
         }
-        sourceDirectories[source] = 'app/mirage';
+        return source;
       }
     }
   }