fix(input): vendor-prefix ::placeholder (#2547)

Author: mmalerba

src/lib/input/input-container.scss

@@ -15,10 +15,19 @@ md-input-container {
 }
 
 .md-input-element {
+  // Note that we can't use something like visibility: hidden or
+  // display: none, because IE ends up preventing the user from
+  // focusing the input altogether.
   &::placeholder {
-    // Note that we can't use something like visibility: hidden or
-    // display: none, because IE ends up preventing the user from
-    // focusing the input altogether.
+    color: transparent;
+  }
+  &::-moz-placeholder {
+    color: transparent;
+  }
+  &::-webkit-input-placeholder {
+    color: transparent;
+  }
+  &:-ms-input-placeholder {
     color: transparent;
   }