Add required field mixin

bower.json

@@ -9,6 +9,7 @@
     "mixins/field-button.html",
     "mixins/menu-overlay.html",
     "mixins/overlay.html",
+    "mixins/required-field.html",
     "color.html",
     "font-icons.html",
     "shadow.html",

mixins/required-field.html

@@ -0,0 +1,55 @@
+<link rel="import" href="../color.html">
+
+<dom-module id="material-required-field">
+  <template>
+    <style>
+      [part="label"] {
+        display: block;
+        position: absolute;
+        top: 8px;
+        font-size: 1em;
+        line-height: 1;
+        height: 20px;
+        margin-bottom: -4px;
+        white-space: nowrap;
+        overflow-x: hidden;
+        text-overflow: ellipsis;
+        color: var(--material-secondary-text-color);
+        transform-origin: 0 75%;
+        transform: scale(0.75);
+      }
+
+      :host([required]) [part="label"]::after {
+        content: " *";
+        color: inherit;
+      }
+
+      :host([invalid]) [part="label"] {
+        color: var(--material-error-text-color);
+      }
+
+      [part="error-message"] {
+        font-size: .75em;
+        line-height: 1;
+        color: var(--material-error-text-color);
+        margin-top: 6px;
+      }
+
+      :host(:not([invalid])) [part="error-message"] {
+        margin-top: 0;
+        max-height: 0;
+        overflow: hidden;
+      }
+
+      :host([invalid]) [part="error-message"] {
+        animation: reveal 0.2s;
+      }
+
+      @keyframes reveal {
+        0% {
+          opacity: 0;
+        }
+      }
+    </style>
+  </template>
+</dom-module>