fix(select): unable to set a tabindex

Fixes users not being able to override the `tabIndex` on `md-select`.

Fixes angular#3474.

src/lib/select/select.spec.ts

@@ -1081,10 +1081,17 @@ describe('MdSelect', () => {
         expect(select.getAttribute('aria-label')).toEqual('Food');
       });
 
-      it('should set the tabindex of the select to 0', () => {
+      it('should set the tabindex of the select to 0 by default', () => {
         expect(select.getAttribute('tabindex')).toEqual('0');
       });
 
+      it('should be able to override the tabindex', () => {
+        fixture.componentInstance.tabIndexOverride = 3;
+        fixture.detectChanges();
+
+        expect(select.getAttribute('tabindex')).toBe('3');
+      });
+
       it('should set aria-required for required selects', () => {
         expect(select.getAttribute('aria-required'))
           .toEqual('false', `Expected aria-required attr to be false for normal selects.`);
@@ -1583,7 +1590,8 @@ describe('MdSelect', () => {
   selector: 'basic-select',
   template: `
     <div [style.height.px]="heightAbove"></div>
-    <md-select placeholder="Food" [formControl]="control" [required]="isRequired">
+    <md-select placeholder="Food" [formControl]="control" [required]="isRequired"
+      [tabIndex]="tabIndexOverride">
       <md-option *ngFor="let food of foods" [value]="food.value" [disabled]="food.disabled">
         {{ food.viewValue }}
       </md-option>
@@ -1606,6 +1614,7 @@ class BasicSelect {
   isRequired: boolean;
   heightAbove = 0;
   heightBelow = 0;
+  tabIndexOverride: number;
 
   @ViewChild(MdSelect) select: MdSelect;
   @ViewChildren(MdOption) options: QueryList<MdOption>;

src/lib/select/select.ts

@@ -99,7 +99,7 @@ export type MdSelectFloatPlaceholderType = 'always' | 'never' | 'auto';
   encapsulation: ViewEncapsulation.None,
   host: {
     'role': 'listbox',
-    '[attr.tabindex]': '_getTabIndex()',
+    '[attr.tabindex]': 'tabIndex',
     '[attr.aria-label]': 'placeholder',
     '[attr.aria-required]': 'required.toString()',
     '[attr.aria-disabled]': 'disabled.toString()',
@@ -151,6 +151,9 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
   /** The animation state of the placeholder. */
   private _placeholderState = '';
 
+  /** Tab index for the element. */
+  private _tabIndex: number = 0;
+
   /**
    * The width of the trigger. Must be saved to set the min width of the overlay panel
    * and the width of the selected value.
@@ -266,6 +269,15 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
   }
   private _floatPlaceholder: MdSelectFloatPlaceholderType = 'auto';
 
+  /** Tab index for the select element. */
+  @Input()
+  get tabIndex(): number { return this._disabled ? -1 : this._tabIndex; }
+  set tabIndex(value: number) {
+    if (typeof value !== 'undefined') {
+      this._tabIndex = value;
+    }
+  }
+
   /** Combined stream of all of the child options' change events. */
   get optionSelectionChanges(): Observable<MdOptionSelectionChange> {
     return Observable.merge(...this.options.map(option => option.onSelectionChange));
@@ -452,12 +464,6 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
     }
   }
 
-  /** Returns the correct tabindex for the select depending on disabled state. */
-  _getTabIndex() {
-    return this.disabled ? '-1' : '0';
-  }
-
-
   /**
    * Sets the scroll position of the scroll container. This must be called after
    * the overlay pane is attached or the scroll container element will not yet be