Suppress error-prone warnings in StringTest.

Change-Id: I1eb330b1b3bffe9d035089844a2cc6e3925b9ee0

user/test/com/google/gwt/emultest/java/lang/StringTest.java

diff --git a/user/test/com/google/gwt/emultest/java/lang/StringTest.java b/user/test/com/google/gwt/emultest/java/lang/StringTest.java
index 3b556ce..5ed47ee 100644
--- a/user/test/com/google/gwt/emultest/java/lang/StringTest.java
+++ b/user/test/com/google/gwt/emultest/java/lang/StringTest.java
@@ -119,6 +119,7 @@
     }
   }
 
+  @SuppressWarnings("ReturnValueIgnored")
   public void testConcat() {
     String abc = String.valueOf(new char[] {'a', 'b', 'c'});
     String def = String.valueOf(new char[] {'d', 'e', 'f'});
@@ -543,6 +544,7 @@
     }
   }
 
+  @SuppressWarnings("ReturnValueIgnored")
   public void testIntern() {
     String s1 = String.valueOf(new char[] {'a', 'b', 'c', 'd', 'e', 'f'});
     String s2 = String.valueOf(new char[] {'a', 'b', 'c', 'd', 'e', 'f'});