Add isDigitsOnly to ShadowTextUtils
diff --git a/src/main/java/com/xtremelabs/robolectric/shadows/ShadowTextUtils.java b/src/main/java/com/xtremelabs/robolectric/shadows/ShadowTextUtils.java
index f1ae08e..d5397ed 100644
--- a/src/main/java/com/xtremelabs/robolectric/shadows/ShadowTextUtils.java
+++ b/src/main/java/com/xtremelabs/robolectric/shadows/ShadowTextUtils.java
@@ -35,4 +35,15 @@
     public static String join(CharSequence delimiter, Object[] tokens) {
         return Join.join((String) delimiter, tokens);
     }
+
+    @Implementation
+    public static boolean isDigitsOnly(CharSequence str) {
+        final int len = str.length();
+        for (int i = 0; i < len; i++) {
+          if (!Character.isDigit(str.charAt(i))) {
+            return false;
+          }
+        }
+        return true;
+    }
 }
diff --git a/src/test/java/com/xtremelabs/robolectric/shadows/TextUtilsTest.java b/src/test/java/com/xtremelabs/robolectric/shadows/TextUtilsTest.java
index 9f49ed8..197c09e 100644
--- a/src/test/java/com/xtremelabs/robolectric/shadows/TextUtilsTest.java
+++ b/src/test/java/com/xtremelabs/robolectric/shadows/TextUtilsTest.java
@@ -31,4 +31,10 @@
       assertThat(TextUtils.join(",", new String[] { "1", "2", "3" }), equalTo("1,2,3"));
       assertThat(TextUtils.join(",", Arrays.asList("1", "2", "3")), equalTo("1,2,3"));
     }
+
+    @Test
+    public void testIsDigitsOnly() throws Exception {
+        assertThat(TextUtils.isDigitsOnly("123456"), equalTo(true));
+        assertThat(TextUtils.isDigitsOnly("124a56"), equalTo(false));
+    }
 }