fix(select): required asterisk should use warn color (#5498)

* When the select is focused and is also required, the asterisk of the select should use the accent palette. Similar to the Material Design input specifications.

Fixes #5496

src/lib/select/_select-theme.scss

@@ -52,6 +52,10 @@
     &.mat-accent {
       @include _mat-select-inner-content-theme($accent);
     }
+
+    &.mat-select-required .mat-select-placeholder::after {
+      color: mat-color($warn);
+    }
   }
 
   .mat-select:focus:not(.mat-select-disabled).mat-warn,

src/lib/select/select.scss

@@ -72,7 +72,7 @@ $mat-select-panel-max-height: 256px !default;
   }
 
   // TODO: Double-check accessibility of this style
-  [aria-required=true] &::after {
+  .mat-select-required &::after {
     content: '*';
   }
 }

src/lib/select/select.spec.ts

@@ -1572,6 +1572,17 @@ describe('MdSelect', () => {
           .toEqual('true', `Expected aria-required attr to be true for required selects.`);
       });
 
+      it('should set the mat-select-required class for required selects', () => {
+        expect(select.classList).not.toContain(
+            'mat-select-required', `Expected the mat-select-required class not to be set.`);
+
+        fixture.componentInstance.isRequired = true;
+        fixture.detectChanges();
+
+        expect(select.classList).toContain(
+          'mat-select-required', `Expected the mat-select-required class to be set.`);
+      });
+
       it('should set aria-invalid for selects that are invalid', () => {
         expect(select.getAttribute('aria-invalid'))
           .toEqual('false', `Expected aria-invalid attr to be false for valid selects.`);

src/lib/select/select.ts

@@ -132,6 +132,7 @@ export const _MdSelectMixinBase = mixinColor(mixinDisabled(MdSelectBase), 'prima
     '[attr.aria-invalid]': '_control?.invalid || "false"',
     '[attr.aria-owns]': '_optionIds',
     '[class.mat-select-disabled]': 'disabled',
+    '[class.mat-select-required]': 'required',
     'class': 'mat-select',
     '(keydown)': '_handleClosedKeydown($event)',
     '(blur)': '_onBlur()',