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/src/common/directives/ng_for.ts

@@ -9,11 +9,12 @@ import {
   EmbeddedViewRef,
   TrackByFn
 } from 'angular2/core';
-import {isPresent, isBlank} from 'angular2/src/facade/lang';
+import {isPresent, isBlank, stringify, getTypeNameForDebugging} from 'angular2/src/facade/lang';
 import {
   DefaultIterableDiffer,
   CollectionChangeRecord
 } from "../../core/change_detection/differs/default_iterable_differ";
+import {BaseException} from "../../facade/exceptions";
 
 /**
  * The `NgFor` directive instantiates a template once per item from an iterable. The context for
@@ -77,7 +78,12 @@ export class NgFor implements DoCheck {
   set ngForOf(value: any) {
     this._ngForOf = value;
     if (isBlank(this._differ) && isPresent(value)) {
+      try {
         this._differ = this._iterableDiffers.find(value).create(this._cdr, this._ngForTrackBy);
+      } catch (e) {
+        throw new BaseException(
+            `Cannot find a differ supporting object '${value}' of type '${getTypeNameForDebugging(value)}'. NgFor only supports binding to Iterables such as Arrays.`);
+      }
     }
   }
 

modules/angular2/src/core/change_detection/differs/iterable_differs.ts

@@ -1,4 +1,4 @@
-import {isBlank, isPresent, CONST} from 'angular2/src/facade/lang';
+import {isBlank, isPresent, CONST, getTypeNameForDebugging} from 'angular2/src/facade/lang';
 import {BaseException} from 'angular2/src/facade/exceptions';
 import {ListWrapper} from 'angular2/src/facade/collection';
 import {ChangeDetectorRef} from '../change_detector_ref';
@@ -86,7 +86,8 @@ export class IterableDiffers {
     if (isPresent(factory)) {
       return factory;
     } else {
-      throw new BaseException(`Cannot find a differ supporting object '${iterable}'`);
+      throw new BaseException(
+          `Cannot find a differ supporting object '${iterable}' of type '${getTypeNameForDebugging(iterable)}'`);
     }
   }
 }

modules/angular2/src/facade/lang.dart

@@ -5,7 +5,7 @@ import 'dart:math' as math;
 import 'dart:convert' as convert;
 import 'dart:async' show Future, Zone;
 
-String getTypeNameForDebugging(Type type) => type.toString();
+String getTypeNameForDebugging(Object type) => type.toString();
 
 class Math {
   static final _random = new math.Random();

modules/angular2/src/facade/lang.ts

@@ -62,7 +62,10 @@ export interface Type extends Function {}
 export interface ConcreteType extends Type { new (...args): any; }
 
 export function getTypeNameForDebugging(type: Type): string {
+  if (type['name']) {
     return type['name'];
+  }
+  return typeof type;
 }
 
 

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)