feat(elevation): Add elevation core styles

This commit replaces `_shadows.scss` within `src/core/styles` with
`_elevation.scss`, according to the proposed design outlined
[here](https://goo.gl/Kq0k9Z). All components that were dependent on the
old shadows have been modified to use elevation. Note however that none
of the values in those old components have been changed; it's simply a
to the new API.

Part of #48.

Author: traviskaufman

src/components/button/_button-base.scss

@@ -1,6 +1,6 @@
 
 @import "variables";
-@import "shadows";
+@import "elevation";
 
 // TODO(jelbourn): This goes away.
 @import "default-theme";
@@ -70,20 +70,20 @@ $md-mini-fab-padding: 8px !default;
 %md-raised-button {
   @extend %md-button-base;
 
+  @include md-elevation(1);
   @include md-button-theme('color', default-contrast);
   @include md-button-theme('background-color');
 
   background-color: md-color($md-background, background);
-  box-shadow: $md-shadow-bottom-z-1;
   // Force hardware acceleration.
   transform: translate3d(0, 0, 0);
 
   // Animation.
   transition: background $swift-ease-out-duration $swift-ease-out-timing-function,
-  box-shadow $swift-ease-out-duration $swift-ease-out-timing-function;
+              md-elevation-transition-property-value();
 
   &:active {
-    box-shadow: $md-shadow-bottom-z-2;
+    @include md-elevation(2);
   }
 
   &.md-button-focus {

src/components/card/card.scss

@@ -1,6 +1,6 @@
 
 @import "variables";
-@import "shadows";
+@import "elevation";
 @import "default-theme";   // TODO: Remove this
 
 $md-card-default-padding: 24px !default;
@@ -9,18 +9,18 @@ $md-card-border-radius: 2px !default;
 $md-card-header-size: 40px !default;
 
 md-card {
+  @include md-elevation(1);
+  @include md-elevation-transition;
   display: block;
   position: relative;
   padding: $md-card-default-padding;
   border-radius: $md-card-border-radius;
-  box-shadow: $md-shadow-bottom-z-1;
   font-family: $md-font-family;
   background: md-color($md-background, card);
 }
 
 md-card:hover {
-  box-shadow: $md-shadow-bottom-z-2;
-  transition: $swift-ease-in;
+  @include md-elevation(2);
 }
 
 .md-card-flat {
@@ -30,7 +30,7 @@ md-card:hover {
 // base styles for each card section preset (md-card-content, etc)
 %md-card-section-base {
   display: block;
-  margin-bottom: 16px;  
+  margin-bottom: 16px;
 }
 
 md-card-title {
@@ -76,7 +76,7 @@ md-card-footer {
 md-card-actions {
   [md-button], [md-raised-button] {
     margin: 0 4px;
-  } 
+  }
 }
 
 /* HEADER STYLES */
@@ -175,7 +175,7 @@ md-card-title-group {
   margin-bottom: 0;
 }
 
-// if main image is on top, need to place it flush against top 
+// if main image is on top, need to place it flush against top
 // (by stripping card's default 24px padding)
 [md-card-image]:first-child {
   margin-top: -24px;
@@ -208,7 +208,7 @@ md-card-header md-card-subtitle:not(:first-child) {
   margin-top: -8px;
 }
 
-// xl image should always have 16px on top. 
+// xl image should always have 16px on top.
 // when it's the first el, it'll need to remove 8px from default card padding of 24px
 .md-card > [md-card-xl-image]:first-child{
   margin-top: -8px;

src/components/sidenav/sidenav.scss

@@ -1,7 +1,7 @@
 @import "default-theme";
 @import "mixins";
 @import "variables";
-@import "shadows";
+@import "elevation";
 
 
 // We use invert() here to have the darken the background color expected to be used. If the
@@ -31,14 +31,14 @@ $md-sidenav-push-background-color: md-color($md-background, dialog) !default;
     will-change: transform;
   }
   &.md-sidenav-opening {
+    @include md-elevation(1);
     visibility: visible;
     transform: translateX($open);
     will-change: transform;
-    box-shadow: $md-shadow-bottom-z-1;
   }
   &.md-sidenav-opened {
+    @include md-elevation(1);
     transform: translateX($open);
-    box-shadow: $md-shadow-bottom-z-1;
   }
 }
 

src/core/style/_elevation.scss

@@ -0,0 +1,179 @@
+/**
+ * A collection of mixins and CSS classes that can be used to apply elevation to a material
+ * element.
+ * See: https://www.google.com/design/spec/what-is-material/elevation-shadows.html
+ * Examples:
+ *
+ *
+ * .md-foo {
+ *   @include $md-elevation(2);
+ *
+ *   &:active {
+ *     @include $md-elevation(8);
+ *   }
+ * }
+ *
+ * <div id="external-card" class="md-elevation-z2"><p>Some content</p></div>
+ *
+ * For an explanation of the design behind how elevation is implemented, see the design doc at
+ * https://goo.gl/Kq0k9Z.
+ */
+
+// Colors for umbra, penumbra, and ambient shadows. As described in the design doc, each elevation
+// level is created using a set of 3 shadow values, one for umbra (the shadow representing the
+// space completely obscured by an object relative to its light source), one for penumbra (the
+// space partially obscured by an object), and one for ambient (the space which contains the object
+// itself). For a further explanation of these terms and their meanings, see
+// https://en.wikipedia.org/wiki/Umbra,_penumbra_and_antumbra.
+
+$_umbra-color: rgba(black, 0.2);
+$_penumbra-color: rgba(black, 0.14);
+$_ambient-color: rgba(black, 0.12);
+
+// Maps for the different shadow sets and their values within each z-space. These values were
+// created by taking a few reference shadow sets created by Google's Designers and interpolating
+// all of the values between them.
+
+$_umbra-elevation-map: (
+  0: '0px 0px 0px 0px #{$_umbra-color}',
+  1: '0px 2px 1px -1px #{$_umbra-color}',
+  2: '0px 3px 1px -2px #{$_umbra-color}',
+  3: '0px 3px 3px -2px #{$_umbra-color}',
+  4: '0px 2px 4px -1px #{$_umbra-color}',
+  5: '0px 3px 5px -1px #{$_umbra-color}',
+  6: '0px 3px 5px -1px #{$_umbra-color}',
+  7: '0px 4px 5px -2px #{$_umbra-color}',
+  8: '0px 5px 5px -3px #{$_umbra-color}',
+  9: '0px 5px 6px -3px #{$_umbra-color}',
+  10: '0px 6px 6px -3px #{$_umbra-color}',
+  11: '0px 6px 7px -4px #{$_umbra-color}',
+  12: '0px 7px 8px -4px #{$_umbra-color}',
+  13: '0px 7px 8px -4px #{$_umbra-color}',
+  14: '0px 7px 9px -4px #{$_umbra-color}',
+  15: '0px 8px 9px -5px #{$_umbra-color}',
+  16: '0px 8px 10px -5px #{$_umbra-color}',
+  17: '0px 8px 11px -5px #{$_umbra-color}',
+  18: '0px 9px 11px -5px #{$_umbra-color}',
+  19: '0px 9px 12px -6px #{$_umbra-color}',
+  20: '0px 10px 13px -6px #{$_umbra-color}',
+  21: '0px 10px 13px -6px #{$_umbra-color}',
+  22: '0px 10px 14px -6px #{$_umbra-color}',
+  23: '0px 11px 14px -7px #{$_umbra-color}',
+  24: '0px 11px 15px -7px #{$_umbra-color}'
+);
+
+$_penumbra-elevation-map: (
+  0: '0px 0px 0px 0px #{$_penumbra-color}',
+  1: '0px 1px 1px 0px #{$_penumbra-color}',
+  2: '0px 2px 2px 0px #{$_penumbra-color}',
+  3: '0px 3px 4px 0px #{$_penumbra-color}',
+  4: '0px 4px 5px 0px #{$_penumbra-color}',
+  5: '0px 5px 8px 0px #{$_penumbra-color}',
+  6: '0px 6px 10px 0px #{$_penumbra-color}',
+  7: '0px 7px 10px 1px #{$_penumbra-color}',
+  8: '0px 8px 10px 1px #{$_penumbra-color}',
+  9: '0px 9px 12px 1px #{$_penumbra-color}',
+  10: '0px 10px 14px 1px #{$_penumbra-color}',
+  11: '0px 11px 15px 1px #{$_penumbra-color}',
+  12: '0px 12px 17px 2px #{$_penumbra-color}',
+  13: '0px 13px 19px 2px #{$_penumbra-color}',
+  14: '0px 14px 21px 2px #{$_penumbra-color}',
+  15: '0px 15px 22px 2px #{$_penumbra-color}',
+  16: '0px 16px 24px 2px #{$_penumbra-color}',
+  17: '0px 17px 26px 2px #{$_penumbra-color}',
+  18: '0px 18px 28px 2px #{$_penumbra-color}',
+  19: '0px 19px 29px 2px #{$_penumbra-color}',
+  20: '0px 20px 31px 3px #{$_penumbra-color}',
+  21: '0px 21px 33px 3px #{$_penumbra-color}',
+  22: '0px 22px 35px 3px #{$_penumbra-color}',
+  23: '0px 23px 36px 3px #{$_penumbra-color}',
+  24: '0px 24px 38px 3px #{$_penumbra-color}'
+);
+
+$_ambient-elevation-map: (
+  0: '0px 0px 0px 0px #{$_ambient-color}',
+  1: '0px 1px 3px 0px #{$_ambient-color}',
+  2: '0px 1px 5px 0px #{$_ambient-color}',
+  3: '0px 1px 8px 0px #{$_ambient-color}',
+  4: '0px 1px 10px 0px #{$_ambient-color}',
+  5: '0px 1px 14px 0px #{$_ambient-color}',
+  6: '0px 1px 18px 0px #{$_ambient-color}',
+  7: '0px 2px 16px 1px #{$_ambient-color}',
+  8: '0px 3px 14px 2px #{$_ambient-color}',
+  9: '0px 3px 16px 2px #{$_ambient-color}',
+  10: '0px 4px 18px 3px #{$_ambient-color}',
+  11: '0px 4px 20px 3px #{$_ambient-color}',
+  12: '0px 5px 22px 4px #{$_ambient-color}',
+  13: '0px 5px 24px 4px #{$_ambient-color}',
+  14: '0px 5px 26px 4px #{$_ambient-color}',
+  15: '0px 6px 28px 5px #{$_ambient-color}',
+  16: '0px 6px 30px 5px #{$_ambient-color}',
+  17: '0px 6px 32px 5px #{$_ambient-color}',
+  18: '0px 7px 34px 6px #{$_ambient-color}',
+  19: '0px 7px 36px 6px #{$_ambient-color}',
+  20: '0px 8px 38px 7px #{$_ambient-color}',
+  21: '0px 8px 40px 7px #{$_ambient-color}',
+  22: '0px 8px 42px 7px #{$_ambient-color}',
+  23: '0px 9px 44px 8px #{$_ambient-color}',
+  24: '0px 9px 46px 8px #{$_ambient-color}'
+);
+
+/**
+ * The css property used for elevation. In most cases this should not be changed. It is exposed
+ * as a variable for abstraction / easy use when needing to reference the property directly, for
+ * example in a will-change rule.
+ */
+$md-elevation-property: box-shadow !default;
+
+/** The default duration value for elevation transitions. */
+$md-elevation-transition-duration: 280ms !default;
+
+/** The default easing value for elevation transitions. */
+$md-elevation-transition-timing-function: $md-fast-out-slow-in-timing-function;
+
+/**
+ * Applies the correct css rules to an element to give it the elevation specified by $zValue.
+ * The $zValue must be between 0 and 24.
+ */
+@mixin md-elevation($zValue) {
+  @if type-of($zValue) != number or not unitless($zValue) {
+    @error "$zValue must be a unitless number";
+  }
+  @if $zValue < 0 or $zValue > 24 {
+    @error "$zValue must be between 0 and 24";
+  }
+
+  #{$md-elevation-property}: #{map-get($_umbra-elevation-map, $zValue)},
+                             #{map-get($_penumbra-elevation-map, $zValue)},
+                             #{map-get($_ambient-elevation-map, $zValue)};
+}
+
+/**
+ * Returns a string that can be used as the value for a transition property for elevation.
+ * Calling this function directly is useful in situations where a component needs to transition
+ * more than one property.
+ *
+ * .foo {
+ *   transition: md-elevation-transition-property-value(), opacity 100ms ease;
+ *   will-change: $md-elevation-property, opacity;
+ * }
+ */
+@function md-elevation-transition-property-value(
+    $duration: $md-elevation-transition-duration,
+    $easing: $md-elevation-transition-timing-function) {
+  @return #{$md-elevation-property} #{$duration} #{$easing};
+}
+
+/**
+ * Applies the correct css rules needed to have an element transition between elevations.
+ * This mixin should be applied to elements whose elevation values will change depending on their
+ * context (e.g. when active or disabled).
+ */
+// NOTE(traviskaufman): Both this mixin and the above function use default parameters so they can
+// be used in the same way by clients.
+@mixin md-elevation-transition(
+    $duration: $md-elevation-transition-duration,
+    $easing: $md-elevation-transition-timing-function) {
+  transition: md-elevation-transition-property-value($duration, $easing);
+  will-change: $md-elevation-property;
+}

src/core/style/_shadows.scss

@@ -0,0 +1,12 @@
+// Core styles that can be used to apply material design treatments to any element.
+
+@import 'elevation';
+
+// Provides external CSS classes for each elevation value. Each CSS class is formatted as
+// `md-elevation-z$zValue` where `$zValue` corresponds to the z-space to which the element is
+// elevated.
+@for $zValue from 0 through 24 {
+  .md-elevation-z#{$zValue} {
+    @include md-elevation($zValue);
+  }
+}