diff --git a/modules/di/src/exceptions.js b/modules/di/src/exceptions.js
index 866356e710..15a971dc6d 100644
--- a/modules/di/src/exceptions.js
+++ b/modules/di/src/exceptions.js
@@ -12,6 +12,8 @@ function constructResolvingPath(keys:List) {
   }
 }
 
+export class KeyMetadataError extends Error {}
+
 export class ProviderError extends Error {
   constructor(key:Key, constructResolvingMessage:Function) {
     this.keys = [key];
diff --git a/modules/di/src/injector.js b/modules/di/src/injector.js
index e99b797e79..a20a184189 100644
--- a/modules/di/src/injector.js
+++ b/modules/di/src/injector.js
@@ -44,13 +44,13 @@ export class Injector {
 
 
   _createListOfBindings(flattenBindings):List {
-    var bindings = ListWrapper.createFixedSize(Key.numberOfKeys() + 1);
+    var bindings = ListWrapper.createFixedSize(Key.numberOfKeys + 1);
     MapWrapper.forEach(flattenBindings, (v, keyId) => bindings[keyId] = v);
     return bindings;
   }
 
   _createInstances():List {
-    return ListWrapper.createFixedSize(Key.numberOfKeys() + 1);
+    return ListWrapper.createFixedSize(Key.numberOfKeys + 1);
   }
 
   _getByKey(key:Key, returnPromise:boolean, returnLazy:boolean) {
diff --git a/modules/di/src/key.js b/modules/di/src/key.js
index a69087a73a..057c7be54d 100644
--- a/modules/di/src/key.js
+++ b/modules/di/src/key.js
@@ -1,30 +1,44 @@
+import {KeyMetadataError} from './exceptions';
 import {MapWrapper, Map} from 'facade/collection';
 import {FIELD, int, isPresent} from 'facade/lang';
 
 var _allKeys = MapWrapper.create();
-var _id:int = 0;
 
 export class Key {
   @FIELD('final token')
   @FIELD('final id:int')
+  @FIELD('metadata:Object')
   constructor(token, id:int) {
     this.token = token;
     this.id = id;
+    this.metadata = null;
   }
 
-  static get(token) {
+  static get(token):Key {
     if (token instanceof Key) return token;
 
     if (MapWrapper.contains(_allKeys, token)) {
       return MapWrapper.get(_allKeys, token);
     }
 
-    var newKey = new Key(token, ++_id);
+    var newKey = new Key(token, Key.numberOfKeys);
     MapWrapper.set(_allKeys, token, newKey);
     return newKey;
   }
 
-  static numberOfKeys() {
-    return _id;
+  static setMetadata(key:Key, metadata):Key {
+    if (isPresent(key.metadata) && key.metadata !== metadata) {
+      throw new KeyMetadataError();
+    }
+    key.metadata = metadata;
+    return key;
+  }
+
+  static clear() {
+    _allKeys = MapWrapper.create();
+  }
+
+  static get numberOfKeys():int {
+    return MapWrapper.size(_allKeys);
   }
 }
diff --git a/modules/di/test/di/key_spec.js b/modules/di/test/di/key_spec.js
index cebf492131..34eed487f4 100644
--- a/modules/di/test/di/key_spec.js
+++ b/modules/di/test/di/key_spec.js
@@ -1,8 +1,12 @@
-import {describe, it, expect} from 'test_lib/test_lib';
+import {describe, it, expect, beforeEach} from 'test_lib/test_lib';
 import {Key} from 'di/di';
 
 export function main() {
   describe("key", function () {
+    beforeEach(function () {
+      Key.clear();
+    });
+
     it('should be equal to another key if type is the same', function () {
       expect(Key.get('car')).toBe(Key.get('car'));
     });
@@ -14,5 +18,32 @@ export function main() {
     it('should return the passed in key', function () {
       expect(Key.get(Key.get('car'))).toBe(Key.get('car'));
     });
+
+    describe("metadata", function () {
+      it("should assign metadata to a key", function () {
+        var key = Key.get('car');
+
+        Key.setMetadata(key, "meta");
+
+        expect(key.metadata).toEqual("meta");
+      });
+
+      it("should allow assigning the same metadata twice", function () {
+        var key = Key.get('car');
+
+        Key.setMetadata(key, "meta");
+        Key.setMetadata(key, "meta");
+
+        expect(key.metadata).toEqual("meta");
+      });
+
+      it("should throw when assigning different metadata", function () {
+        var key = Key.get('car');
+
+        Key.setMetadata(key, "meta1");
+
+        expect(() => Key.setMetadata(key, "meta2")).toThrowError();
+      });
+    });
   });
 }
\ No newline at end of file
diff --git a/modules/facade/src/collection.dart b/modules/facade/src/collection.dart
index 46f9e31b90..8844b25d2b 100644
--- a/modules/facade/src/collection.dart
+++ b/modules/facade/src/collection.dart
@@ -11,6 +11,7 @@ class MapWrapper {
   static forEach(m, fn) {
     m.forEach((k,v) => fn(v,k));
   }
+  static int size(m) {return m.length;}
 }
 
 class ListWrapper {
diff --git a/modules/facade/src/collection.es6 b/modules/facade/src/collection.es6
index 647caa1c05..2195127f10 100644
--- a/modules/facade/src/collection.es6
+++ b/modules/facade/src/collection.es6
@@ -10,6 +10,7 @@ export class MapWrapper {
   static forEach(m, fn) {
     m.forEach(fn);
   }
+  static size(m) {return m.size;}
 }
 
 
diff --git a/modules/test_lib/src/test_lib.dart b/modules/test_lib/src/test_lib.dart
index 507e27c89d..2b7d465f66 100644
--- a/modules/test_lib/src/test_lib.dart
+++ b/modules/test_lib/src/test_lib.dart
@@ -15,7 +15,7 @@ Expect expect(actual, [matcher]) {
 class Expect extends gns.Expect {
   Expect(actual) : super(actual);
 
-  void toThrowError(message) => this.toThrowWith(message: message);
+  void toThrowError([message=""]) => this.toThrowWith(message: message);
   void toBePromise() => _expect(actual is Future, equals(true));
   Function get _expect => gns.guinness.matchers.expect;
 }