[debugger] debug-evaluate should not not modify local values.

test/mjsunit/es6/debug-blockscopes.js

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 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --expose-debug-as debug --allow-natives-syntax
+// Flags: --debug-eval-readonly-locals
 // The functions used for testing backtraces. They are at the top to make the
 // testing of source line/column easier.
 
 "use strict";
 
 // Get the Debug object exposed from the debug context global object.
 var Debug = debug.Debug;
 
 var test_name;
 var listener_delegate;
@@ skipping 479 matching lines @@
 EndTest();
 
 
 // Block scopes shadowing
 BeginTest("Block scopes shadowing 1");
 function shadowing_1() {
   let i = 0;
   {
     let i = 5;
     debugger;
-    assertEqualsUnlessOptimized(27, i, shadowing_1);
   }
   assertEquals(0, i);
-  debugger;
-  assertEqualsUnlessOptimized(27, i, shadowing_1);
 }
 
 listener_delegate = function (exec_state) {
-  exec_state.frame(0).evaluate("i = 27");
+  assertEqualsUnlessOptimized(5, exec_state.frame(0).evaluate("i").value());
 }
 shadowing_1();
 EndTest();
 
 
 // Block scopes shadowing
 BeginTest("Block scopes shadowing 2");
 function shadowing_2() {
   let i = 0;
   {
     let j = 5;
     debugger;
-    assertEqualsUnlessOptimized(27, j, shadowing_2);
   }
-  assertEqualsUnlessOptimized(0, i, shadowing_2);
 }
 
 listener_delegate = function (exec_state) {
-  exec_state.frame(0).evaluate("j = 27");
+  assertEqualsUnlessOptimized(0, exec_state.frame(0).evaluate("i").value());
+  assertEqualsUnlessOptimized(5, exec_state.frame(0).evaluate("j").value());
 }
 shadowing_2();
 EndTest();