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

test/mjsunit/debug-evaluate-locals.js

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 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Flags: --expose-debug-as debug
+// Flags: --expose-debug-as debug --debug-eval-readonly-locals
 // Get the Debug object exposed from the debug context global object.
 Debug = debug.Debug
 
 listenerComplete = false;
 exception = false;
 
 
 function h() {
   var a = 1;
   var b = 2;
@@ skipping 81 matching lines @@
       assertEquals(5, exec_state.frame(0).evaluate('eval').value());
       assertEquals(3, exec_state.frame(1).evaluate('a').value());
       // Reference error because g does not reference b.
       assertThrows(() => exec_state.frame(1).evaluate('b'), ReferenceError);
       assertEquals("function",
                    typeof exec_state.frame(1).evaluate('eval').value());
       assertEquals(5, exec_state.frame(2).evaluate('a').value());
       assertEquals(6, exec_state.frame(2).evaluate('b').value());
       assertEquals("function",
                    typeof exec_state.frame(2).evaluate('eval').value());
+      // Assignments to local variables only have temporary effect.
       assertEquals("foo",
                    exec_state.frame(0).evaluate('a = "foo"').value());
       assertEquals("bar",
                    exec_state.frame(1).evaluate('a = "bar"').value());
       // Indicate that all was processed.
       listenerComplete = true;
     }
   } catch (e) {
     exception = e;
     print("Caught something. " + e + " " + e.stack);
   };
 };
 
 // Add the debug event listener.
 Debug.setListener(listener);
 
 var f_result = f();
 
-assertEquals('foobar', f_result);
+assertEquals(4, f_result);
 
 // Make sure that the debug event listener was invoked.
 assertFalse(exception, "exception in listener")
 assertTrue(listenerComplete);