fix(colors): failing unit tests against Edge (#9876)

IE and Edge convert RGB colors to RGBA automatically, which was being handled by the `colors` unit tests via browser sniffing in IE. The browser sniffing doesn't work in Edge, which was causing the tests to fail. This hasn't been noticed until now since we've only been running the tests against IE 11.
These changes switch to using feature detection to check whether the browser converts to RGBA.

Author: crisbeto

src/components/colors/colors.spec.js

@@ -3,6 +3,7 @@ describe('md-colors', function () {
   var $mdColorPalette, $mdTheming;
   var supplant, scope;
   var compiledElements = [];
+  var usesRGBA;
 
   beforeEach(module('material.components.colors', function ($mdThemingProvider) {
     $mdThemingProvider.theme('myTheme')
@@ -17,6 +18,7 @@ describe('md-colors', function () {
     $mdTheming = $injector.get('$mdTheming');
     supplant = $injector.get('$mdUtil').supplant;
     scope = $rootScope.$new();
+    checkColorMode();
   }));
 
   afterEach(function() {
@@ -27,14 +29,8 @@ describe('md-colors', function () {
     compiledElements = [];
   });
 
-  // documentMode is an only-IE property, which confirms that the specs are currently running inside
-  // of an Internet Explorer.
-  var isIE = !!window.document.documentMode;
-
   function buildColor(red, green, blue, opacity) {
-    // Once the current test browser is IE11, we always have to build a RGBA string, because
-    // IE11 automatically transforms all RGB colors into RGBA.
-    if (angular.isDefined(opacity) || isIE) {
+    if (angular.isDefined(opacity) || usesRGBA) {
       return supplant('rgba({0}, {1}, {2}, {3})', [red, green, blue, opacity || 1]);
     } else {
       return supplant('rgb({0}, {1}, {2})', [red, green, blue]);
@@ -48,6 +44,15 @@ describe('md-colors', function () {
     return element;
   }
 
+  // Checks whether the current browser uses RGB or RGBA colors. This is
+  // necessary, because IE and Edge automatically convert RGB colors to RGBA.
+  function checkColorMode() {
+    if (angular.isUndefined(usesRGBA)) {
+      var testerElement = compile('<div md-colors="{background: \'red\'}" >');
+      usesRGBA = testerElement[0].style.background.indexOf('rgba') === 0;
+    }
+  }
+
   describe('directive', function () {
 
     function createElement(scope, options) {