Improve error message when 'del' keyword is used

--
MOS_MIGRATED_REVID=135784854
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java b/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
index 36ecfe1..b761450 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/ParserTest.java
@@ -401,6 +401,13 @@
   }
 
   @Test
+  public void testDel() {
+    setFailFast(false);
+    parseExpression("del d['a']");
+    assertContainsError("'del' not supported, use '.pop()' to delete");
+  }
+
+  @Test
   public void testTupleAssign() {
     List<Statement> statements = parseFile("list[0] = 5; dict['key'] = value\n");
     assertThat(statements).hasSize(2);