diff --git a/tools/broccoli/tree-differ.spec.ts b/tools/broccoli/tree-differ.spec.ts
new file mode 100644
index 0000000000..93998714b0
--- /dev/null
+++ b/tools/broccoli/tree-differ.spec.ts
@@ -0,0 +1,190 @@
+/// <reference path="../typings/node/node.d.ts" />
+/// <reference path="../typings/jasmine/jasmine.d.ts" />
+
+let mockfs = require('mock-fs');
+import fs = require('fs');
+import TreeDiffer = require('./tree-differ');
+
+
+describe('TreeDiffer', () => {
+
+  afterEach(() => mockfs.restore());
+
+
+  describe('diff of changed files', () => {
+
+    it('should list all files but no directories during the first diff', () => {
+      let testDir = {
+        'dir1': {
+          'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+          'file-2.txt': mockfs.file({content: 'file-2.txt content', mtime: new Date(1000)}),
+          'subdir-1': {
+            'file-1.1.txt': mockfs.file({content: 'file-1.1.txt content', mtime: new Date(1000)})
+          },
+          'empty-dir': {}
+        }
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('dir1');
+
+      let diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths)
+          .toEqual(['file-1.txt', 'file-2.txt', 'subdir-1/file-1.1.txt']);
+
+      expect(diffResult.removedPaths).toEqual([]);
+    });
+
+
+    it('should return empty diff if nothing has changed', () => {
+      let testDir = {
+        'dir1': {
+          'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+          'file-2.txt': mockfs.file({content: 'file-2.txt content', mtime: new Date(1000)}),
+          'subdir-1': {
+            'file-1.1.txt': mockfs.file({content: 'file-1.1.txt content', mtime: new Date(1000)})
+          },
+        }
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('dir1');
+
+      let diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths).not.toEqual([]);
+      expect(diffResult.removedPaths).toEqual([]);
+
+      diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths).toEqual([]);
+      expect(diffResult.removedPaths).toEqual([]);
+    });
+
+
+    it('should list only changed files during the subsequent diffs', () => {
+      let testDir = {
+        'dir1': {
+          'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+          'file-2.txt': mockfs.file({content: 'file-2.txt content', mtime: new Date(1000)}),
+          'subdir-1': {
+            'file-1.1.txt':
+                mockfs.file({content: 'file-1.1.txt content', mtime: new Date(1000)})
+          }
+        }
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('dir1');
+
+      let diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths)
+          .toEqual(['file-1.txt', 'file-2.txt', 'subdir-1/file-1.1.txt']);
+
+      // change two files
+      testDir['dir1']['file-1.txt'] = mockfs.file({content: 'new content', mtime: new Date(1000)});
+      testDir['dir1']['subdir-1']['file-1.1.txt'] =
+          mockfs.file({content: 'file-1.1.txt content', mtime: new Date(9999)});
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+
+      expect(diffResult.changedPaths).toEqual(['file-1.txt', 'subdir-1/file-1.1.txt']);
+
+      expect(diffResult.removedPaths).toEqual([]);
+
+      // change one file
+      testDir['dir1']['file-1.txt'] = mockfs.file({content: 'super new', mtime: new Date(1000)});
+      mockfs(testDir);
+
+      diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual(['file-1.txt']);
+    });
+  });
+
+  describe('diff of new files', () => {
+
+    it('should detect file additions and report them as changed files', () => {
+      let testDir = {
+        'dir1':
+            {'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)})}
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('dir1');
+      differ.diffTree();
+
+      testDir['dir1']['file-2.txt'] = 'new file';
+      mockfs(testDir);
+
+      let diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual(['file-2.txt']);
+    });
+  });
+
+
+  it('should detect file additions mixed with file changes', () => {
+    let testDir = {
+      'dir1': {'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)})}
+    };
+    mockfs(testDir);
+
+    let differ = new TreeDiffer('dir1');
+    differ.diffTree();
+
+    testDir['dir1']['file-1.txt'] = 'new content';
+    testDir['dir1']['file-2.txt'] = 'new file';
+    mockfs(testDir);
+
+    let diffResult = differ.diffTree();
+    expect(diffResult.changedPaths).toEqual(['file-1.txt', 'file-2.txt']);
+  });
+
+
+  describe('diff of removed files', () => {
+
+    it('should detect file removals and report them as removed files', () => {
+      let testDir = {
+        'dir1':
+            {'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)})}
+      };
+      mockfs(testDir);
+
+      let differ = new TreeDiffer('dir1');
+      differ.diffTree();
+
+      delete testDir['dir1']['file-1.txt'];
+      mockfs(testDir);
+
+      let diffResult = differ.diffTree();
+      expect(diffResult.changedPaths).toEqual([]);
+      expect(diffResult.removedPaths).toEqual(['file-1.txt']);
+    });
+  });
+
+
+  it('should detect file removals mixed with file changes and additions', () => {
+    let testDir = {
+      'dir1': {
+        'file-1.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)}),
+        'file-2.txt': mockfs.file({content: 'file-1.txt content', mtime: new Date(1000)})
+      }
+    };
+
+    mockfs(testDir);
+
+    let differ = new TreeDiffer('dir1');
+    differ.diffTree();
+
+    testDir['dir1']['file-1.txt'] = 'changed content';
+    delete testDir['dir1']['file-2.txt'];
+    testDir['dir1']['file-3.txt'] = 'new content';
+    mockfs(testDir);
+
+    let diffResult = differ.diffTree();
+    expect(diffResult.changedPaths).toEqual(['file-1.txt', 'file-3.txt']);
+    expect(diffResult.removedPaths).toEqual(['file-2.txt']);
+  });
+});
diff --git a/tools/broccoli/tree-differ.ts b/tools/broccoli/tree-differ.ts
new file mode 100644
index 0000000000..0762adbb17
--- /dev/null
+++ b/tools/broccoli/tree-differ.ts
@@ -0,0 +1,110 @@
+/// <reference path="../typings/node/node.d.ts" />
+
+import fs = require('fs');
+import path = require('path');
+
+
+export = TreeDiffer;
+
+class TreeDiffer {
+  private fingerprints: {[key: string]: string} = Object.create(null);
+  private nextFingerprints: {[key: string]: string} = Object.create(null);
+  private rootDirName: string;
+
+  constructor(private rootPath: string) { this.rootDirName = path.basename(rootPath); }
+
+
+  public diffTree(): DiffResult {
+    let result = new DiffResult(this.rootDirName);
+    this.dirtyCheckPath(this.rootPath, result);
+    this.detectDeletionsAndUpdateFingerprints(result);
+    result.endTime = Date.now();
+    return result;
+  }
+
+
+  private dirtyCheckPath(rootDir: string, result: DiffResult) {
+    fs.readdirSync(rootDir).forEach((segment) => {
+      let absolutePath = path.join(rootDir, segment);
+      let pathStat = fs.statSync(absolutePath);
+
+      if (pathStat.isDirectory()) {
+        result.directoriesChecked++;
+        this.dirtyCheckPath(absolutePath, result);
+      } else {
+        result.filesChecked++;
+        if (this.isFileDirty(absolutePath, pathStat)) {
+          result.changedPaths.push(path.relative(this.rootPath, absolutePath));
+        }
+      }
+    });
+
+    return result;
+  }
+
+
+  private isFileDirty(path: string, stat: fs.Stats): boolean {
+    let oldFingerprint = this.fingerprints[path];
+    let newFingerprint = `${stat.mtime.getTime()} # ${stat.size}`;
+
+    this.nextFingerprints[path] = newFingerprint;
+
+    if (oldFingerprint) {
+      this.fingerprints[path] = null;
+
+      if (oldFingerprint === newFingerprint) {
+        // nothing changed
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+
+  private detectDeletionsAndUpdateFingerprints(result: DiffResult) {
+    for (let absolutePath in this.fingerprints) {
+      if (this.fingerprints[absolutePath] !== null) {
+        let relativePath = path.relative(this.rootPath, absolutePath);
+        result.removedPaths.push(relativePath);
+      }
+    }
+
+    this.fingerprints = this.nextFingerprints;
+    this.nextFingerprints = Object.create(null);
+  }
+}
+
+
+class DiffResult {
+  public filesChecked: number = 0;
+  public directoriesChecked: number = 0;
+  public changedPaths: string[] = [];
+  public removedPaths: string[] = [];
+  public startTime: number = Date.now();
+  public endTime: number = null;
+
+  constructor(public name: string) {}
+
+  toString() {
+    return `${pad(this.name, 40)}, ` +
+           `duration: ${pad(this.endTime - this.startTime, 5)}ms, ` +
+           `${pad(this.changedPaths.length + this.removedPaths.length, 5)} changes detected ` +
+           `(files: ${pad(this.filesChecked, 5)}, directories: ${pad(this.directoriesChecked, 4)})`;
+  }
+
+  log(verbose) {
+    let prefixedPaths =
+        this.changedPaths.map((p) => `* ${p}`).concat(this.removedPaths.map((p) => `- ${p}`));
+    console.log(`Tree diff: ${this}` +
+                ((verbose && prefixedPaths.length) ? ` [\n  ${prefixedPaths.join('\n  ')}\n]` :  ''));
+  }
+}
+
+
+function pad(value, length) {
+  value = '' + value;
+  let whitespaceLength = (value.length < length) ? length - value.length : 0;
+  whitespaceLength = whitespaceLength + 1;
+  return new Array(whitespaceLength).join(' ') + value;
+}