Update to latest Error Prone and fix some newly-flagged problems. (#2426)

gson/src/main/java/com/google/gson/internal/NonNullElementWrapperList.java

@@ -33,6 +33,7 @@ public class NonNullElementWrapperList<E> extends AbstractList<E> implements Ran
   // Explicitly specify ArrayList as type to guarantee that delegate implements RandomAccess
   private final ArrayList<E> delegate;
 
+  @SuppressWarnings("NonApiType")
   public NonNullElementWrapperList(ArrayList<E> delegate) {
     this.delegate = Objects.requireNonNull(delegate);
   }

gson/src/test/java/com/google/gson/functional/DefaultTypeAdaptersTest.java

@@ -691,6 +691,7 @@ public class DefaultTypeAdaptersTest {
     assertThat(treeSet).contains("Value1");
   }
 
+  @SuppressWarnings("UnnecessaryStringBuilder") // TODO: b/287969247 - remove when EP bug fixed
   @Test
   public void testStringBuilderSerialization() {
     StringBuilder sb = new StringBuilder("abc");

gson/src/test/java/com/google/gson/functional/ExposeFieldsTest.java

@@ -60,11 +60,14 @@ public class ExposeFieldsTest {
     ClassWithExposedFields[] objects = { object1, object2, object3 };
 
     String json = gson.toJson(objects);
-    String expected = new StringBuilder()
-        .append('[').append(object1.getExpectedJson()).append(',')
-        .append(object2.getExpectedJson()).append(',')
-        .append(object3.getExpectedJson()).append(']')
-        .toString();
+    String expected =
+        '['
+            + object1.getExpectedJson()
+            + ','
+            + object2.getExpectedJson()
+            + ','
+            + object3.getExpectedJson()
+            + ']';
 
     assertThat(json).isEqualTo(expected);
   }

pom.xml

@@ -150,7 +150,7 @@
               <path>
                 <groupId>com.google.errorprone</groupId>
                 <artifactId>error_prone_core</artifactId>
-                <version>2.19.1</version>
+                <version>2.20.0</version>
               </path>
             </annotationProcessorPaths>
           </configuration>