feat(debug): replace DebugElement with new Debug DOM

Now, using `ng.probe(element)` in the browser console returns
a DebugElement when in dev mode.

`ComponentFixture#debugElement` also returns a new DebugElement.

Breaking Change:

This is a breaking change for unit tests. The API for the DebugElement
has changed. Now, there is a DebugElement or DebugNode for every node
in the DOM, not only nodes with an ElementRef. `componentViewChildren` is
removed, and `childNodes` is a list of ElementNodes corresponding to every
child in the DOM. `query` no longer takes a scope parameter, since
the entire rendered DOM is included in the `childNodes`.

Before:

```
componentFixture.debugElement.componentViewChildren[0];
```

After
```
// Depending on the DOM structure of your component, the
// index may have changed or the first component child
// may be a sub-child.
componentFixture.debugElement.children[0];
```

Before:

```
debugElement.query(By.css('div'), Scope.all());
```

After:

```
debugElement.query(By.css('div'));
```

Before:

```
componentFixture.debugElement.elementRef;
```

After:

```
componentFixture.elementRef;
```

modules/angular2_material/test/button_spec.ts

@@ -12,9 +12,8 @@ import {
   it,
   xit,
 } from 'angular2/testing_internal';
-import {DebugElement} from 'angular2/src/core/debug/debug_element';
 
-import {Component, View, ViewMetadata, bind, provide} from 'angular2/core';
+import {Component, View, ViewMetadata, bind, provide, DebugElement} from 'angular2/core';
 import {UrlResolver} from 'angular2/compiler';
 
 import {MdButton, MdAnchor} from 'angular2_material/src/components/button/button';
@@ -110,7 +109,8 @@ export function main() {
 
 /** Gets a child DebugElement by tag name. */
 function getChildDebugElement(parent: DebugElement, tagName: string): DebugElement {
-  return parent.query(debugEl => debugEl.nativeElement.tagName.toLowerCase() == tagName);
+  var el = parent.query(debugEl => debugEl.nativeNode.tagName.toLowerCase() == tagName);
+  return <DebugElement>el;
 }
 
 /** Test component that contains an MdButton. */