feat(di): add OpaqueToken to DI

Using `new Object()` as a token causes cryptic errors. OpaqueToken class should be used instead.

modules/di/src/di.js

@@ -4,3 +4,4 @@ export * from './binding';
 export * from './key';
 export * from './module';
 export * from './exceptions';
+export * from './opaque_token';

modules/di/src/opaque_token.js

@@ -0,0 +1,11 @@
+export class OpaqueToken {
+  _desc:string;
+
+  constructor(desc:string){
+    this._desc = `Token(${desc})`;
+  }
+
+  toString() {
+    return this._desc;
+  }
+}