JS backend: added @Nullable to JsNameRef#getName and added assertion to get more information for reproduce #EA-66032

js/js.dart-ast/src/com/google/dart/compiler/backend/js/ast/JsNameRef.java

@@ -7,6 +7,7 @@ package com.google.dart.compiler.backend.js.ast;
 import com.google.dart.compiler.common.Symbol;
 import com.google.dart.compiler.util.AstUtil;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * Represents a JavaScript expression that references a name.
@@ -43,6 +44,7 @@ public final class JsNameRef extends JsExpressionImpl implements HasName {
         return (name == null) ? ident : name.getIdent();
     }
 
+    @Nullable
     @Override
     public JsName getName() {
         return name;