Add critical checks in StringJoiner.

This is a stop gap measure to make tests pass.
As a follow up compiler needs to be fixed to handle
CharSequence.toString properly for null values.

Change-Id: I0b9a50c762b16f9dae0693171bb135bedeb26ff7

user/super/com/google/gwt/emul/java/util/StringJoiner.java

diff --git a/user/super/com/google/gwt/emul/java/util/StringJoiner.java b/user/super/com/google/gwt/emul/java/util/StringJoiner.java
index c93b3b4..0e67640 100644
--- a/user/super/com/google/gwt/emul/java/util/StringJoiner.java
+++ b/user/super/com/google/gwt/emul/java/util/StringJoiner.java
@@ -35,6 +35,13 @@
   }
 
   public StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix) {
+
+    // TODO(dankurka): remove these checks once we have the proper compiler fix
+    // for Charsequence.toString() with null values.
+    checkCriticalNotNull(delimiter, "delimiter");
+    checkCriticalNotNull(prefix, "prefix");
+    checkCriticalNotNull(suffix, "suffix");
+
     this.delimiter = delimiter.toString();
     this.prefix = prefix.toString();
     this.suffix = suffix.toString();