Added @NotNull annotations.

compiler/frontend.java/src/org/jetbrains/jet/lang/resolve/java/kt/JetClassAnnotation.java

@@ -41,6 +41,7 @@ public class JetClassAnnotation extends PsiAnnotationWithFlags {
         signature = getStringAttribute(JvmStdlibNames.JET_CLASS_SIGNATURE, "");
     }
 
+    @NotNull
     public String signature() {
         checkInitialized();
         return signature;

compiler/frontend.java/src/org/jetbrains/jet/lang/resolve/java/kt/KotlinSignatureAnnotation.java

@@ -41,6 +41,7 @@ public class KotlinSignatureAnnotation extends PsiAnnotationWrapper {
         signature = StringUtil.unescapeStringCharacters(getStringAttribute(JvmStdlibNames.KOTLIN_SIGNATURE_VALUE_METHOD, ""));
     }
 
+    @NotNull
     public String signature() {
         checkInitialized();
         return signature;