Add test that range() returns a list

This is useful before we optimize range() to return a lazy object that appears like a list to the user.

RELNOTES: None
PiperOrigin-RevId: 197944773

src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java

@@ -489,6 +489,24 @@ public void testRange() throws Exception {
         .testIfErrorContains("step cannot be 0", "range(2, 3, 0)");
   }
 
+  @Test
+  public void testRangeIsList() throws Exception {
+    // range() may change in the future to a read-only view, but for now it's a list and can be
+    // updated. This test ensures we won't break backward compatibility until we intend to.
+    new BothModesTest()
+        .testStatement("a = range(2); a.append(2); str(a)", "[0, 1, 2]")
+        .testStatement("a = range(2); type(a)", "list");
+    new SkylarkTest()
+        .testStatement(
+            "def f():\n"
+            + "  a = range(2)\n"
+            + "  b = a\n"
+            + "  a += [2]\n"
+            + "  return str(b)\n"
+            + "f()\n",
+            "[0, 1, 2]");
+  }
+
   @Test
   public void testEnumerate() throws Exception {
     new BothModesTest()