fix(ngFor): give more instructive error when binding to non-iterable

Before, you'd get an error like:

```
EXCEPTION: Cannot find a differ supporting object ‘[object Object]’ in [users in UsersCmp@2:14]
```

Now, you get:

```
EXCEPTION: Cannot find a differ supporting object ‘[object Object]’ of type 'Object'. Did you mean to bind ngFor to an Array? in [users in UsersCmp@2:14]
```

modules/angular2/test/common/directives/ng_for_spec.ts

@@ -14,6 +14,7 @@ import {
 } from 'angular2/testing_internal';
 
 import {ListWrapper} from 'angular2/src/facade/collection';
+import {IS_DART} from 'angular2/src/facade/lang';
 import {Component, TemplateRef, ContentChild} from 'angular2/core';
 import {NgFor} from 'angular2/src/common/directives/ng_for';
 import {NgIf} from 'angular2/src/common/directives/ng_if';
@@ -158,6 +159,24 @@ export function main() {
              });
        }));
 
+    if (!IS_DART) {
+      it('should throw on non-iterable ref and suggest using an array',
+         inject([TestComponentBuilder, AsyncTestCompleter], (tcb: TestComponentBuilder, async) => {
+           tcb.overrideTemplate(TestComponent, TEMPLATE)
+               .createAsync(TestComponent)
+               .then((fixture) => {
+                 fixture.debugElement.componentInstance.items = 'whaaa';
+                 try {
+                   fixture.detectChanges()
+                 } catch (e) {
+                   expect(e.message).toContain(
+                       `Cannot find a differ supporting object 'whaaa' of type 'string'. NgFor only supports binding to Iterables such as Arrays.`);
+                   async.done();
+                 }
+               });
+         }));
+    }
+
     it('should throw on ref changing to string',
        inject([TestComponentBuilder, AsyncTestCompleter], (tcb: TestComponentBuilder, async) => {
          tcb.overrideTemplate(TestComponent, TEMPLATE)