feat(slide-toggle): add support for labelPosition

* Adds an option to change the position of the slide-toggle's label.

Closes #2820

src/lib/slide-toggle/slide-toggle.scss

@@ -66,17 +66,30 @@ md-slide-toggle {
   cursor: pointer;
 }
 
+/* If the label should be placed before the thumb then we just change the orders. */
+.md-slide-toggle-label-before {
+  .md-slide-toggle-label { order: 1; }
+  .md-slide-toggle-container { order: 2; }
+}
+
 // Container for the composition of the slide-toggle / switch indicator.
 .md-slide-toggle-container {
   cursor: grab;
   width: $md-slide-toggle-width;
   height: $md-slide-toggle-height;
 
   position: relative;
+}
 
+/* Apply the margin for slide-toggles and revert it for RTL toggles with labelPosition before. */
+[dir="rtl"] .md-slide-toggle-label-before .md-slide-toggle-container, .md-slide-toggle-container {
   margin-right: $md-slide-toggle-spacing;
+  margin-left: 0;
+}
 
-  [dir='rtl'] & {
+/* Switch the margins in RTL mode and also switch it if the labelPosition is set to before. */
+[dir='rtl'], .md-slide-toggle-label-before {
+  .md-slide-toggle-container {
     margin-left: $md-slide-toggle-spacing;
     margin-right: 0;
   }

src/lib/slide-toggle/slide-toggle.spec.ts

@@ -357,6 +357,16 @@ describe('MdSlideToggle', () => {
       expect(document.activeElement).toBe(inputElement);
       expect(slideToggleElement.classList).toContain('md-slide-toggle-focused');
     });
+
+    it('should set a element class if labelPosition is set to before', () => {
+      expect(slideToggleElement.classList).not.toContain('md-slide-toggle-label-before');
+
+      testComponent.labelPosition = 'before';
+      fixture.detectChanges();
+
+      expect(slideToggleElement.classList).toContain('md-slide-toggle-label-before');
+    });
+
   });
 
   describe('custom template', () => {
@@ -584,6 +594,7 @@ function dispatchFocusChangeEvent(eventName: string, element: HTMLElement): void
                      [aria-label]="slideLabel"
                      [aria-labelledby]="slideLabelledBy"
                      [tabIndex]="slideTabindex"
+                     [labelPosition]="labelPosition"
                      (change)="onSlideChange($event)"
                      (click)="onSlideClick($event)">
 
@@ -603,6 +614,7 @@ class SlideToggleTestApp {
   slideLabelledBy: string;
   slideTabindex: number;
   lastEvent: MdSlideToggleChange;
+  labelPosition: string;
 
   onSlideClick(event: Event) {}
   onSlideChange(event: MdSlideToggleChange) {

src/lib/slide-toggle/slide-toggle.ts

@@ -51,6 +51,7 @@ let nextId = 0;
     '[class.md-disabled]': 'disabled',
     // This md-slide-toggle prefix will change, once the temporary ripple is removed.
     '[class.md-slide-toggle-focused]': '_hasFocus',
+    '[class.md-slide-toggle-label-before]': 'labelPosition == "before"',
     '(mousedown)': '_setMousedown()'
   },
   templateUrl: 'slide-toggle.html',
@@ -85,6 +86,9 @@ export class MdSlideToggle implements AfterContentInit, ControlValueAccessor {
   /** Used to specify the tabIndex value for the underlying input element. */
   @Input() tabIndex: number = 0;
 
+  /** Whether the label should appear after or before the checkbox. Defaults to 'after' */
+  @Input() labelPosition: 'before' | 'after' = 'after';
+
   /** Used to set the aria-label attribute on the underlying input element. */
   @Input('aria-label') ariaLabel: string = null;