[ggj][ast][engx] fix: validate non-null elements for SynchronizedStatement (#465)

* fix: swap assertEquals args in JavaWriterVisitorTest to match (expected, actusl) order

* fix: swap assertEquals args in ImportWriterVisitorTest to match (expected, actusl) order

* fix: add node validator to refactor/centralize null element checks

* fix: validate non-null elements for IfStatement

* fix: validate non-null elements for AnonymousClassExpr

* fix: validate non-null elements for BlockStatement

* fix: validate non-null elements for ClassDefinition

* fix: validate non-null elements for ConcreteReference

* fix: validate non-null elements for ForStatement

* fix: validate non-null elements for GeneralForStatement

* fix: validate non-null elements for MethodDefinition

* fix: validate non-null elements for PackageInfoDefinition

* fix: validate non-null elements for SynchronizedStatement

Author: miraleung

src/main/java/com/google/api/generator/engine/ast/SynchronizedStatement.java

@@ -60,6 +60,14 @@ public Builder setBody(Statement... statements) {
     // Private.
     abstract Builder setLock(Expr expr);
 
-    public abstract SynchronizedStatement build();
+    abstract ImmutableList<Statement> body();
+
+    abstract SynchronizedStatement autoBuild();
+
+    public SynchronizedStatement build() {
+      NodeValidator.checkNoNullElements(body(), "body statements", "synchronized statement");
+
+      return autoBuild();
+    }
   }
 }