anchor.diff:        sizes: [80, 20],
         minSize: [200, 60],
       );
  +
  +    final options = <String, dynamic>{
  +      'mode': 'dart',
  +      'lineNumbers': true,
  +      'gutters': <String>['breakpoints'],
  +    };
  +    final codeMirror =
  +        CodeMirror.fromElement(_sourceArea.element, options: options);
  +    codeMirror.setReadOnly(true);
  +    if (isDarkTheme) {
  +      codeMirror.setTheme('darcula');
  +    }
  +    final codeMirrorElement = _sourcePathDiv.element.parent.children[1];
  +    codeMirrorElement.setAttribute('flex', '');
  +
  +    sourceEditor = SourceEditor(codeMirror, debuggerState);
  +
  +    // TODO(#926): Is this necessary?
  +    sourceEditor.setBreakpoints(debuggerState.breakpoints.value);
  +    debuggerState.breakpoints.addListener(() {
  +      sourceEditor.setBreakpoints(debuggerState.breakpoints.value);
  +    });
     }
   
     @override
  
positive.code:     stepIn.click(() => debuggerState.stepIn());
      stepOut.click(() => debuggerState.stepOut());
  
      final Map<String, dynamic> options = <String, dynamic>{
        'mode': 'dart',
        'lineNumbers': true,
        'gutters': <String>['breakpoints'],
      };
      final CodeMirror codeMirror =
          CodeMirror.fromElement(_sourceArea.element, options: options);
      codeMirror.setReadOnly(true);
      if (isDarkTheme) {
        codeMirror.setTheme('darcula');
      }
      final codeMirrorElement = _sourcePathDiv.element.parent.children[1];
      codeMirrorElement.setAttribute('flex', '');
  
      sourceEditor = SourceEditor(codeMirror, debuggerState);
  
      // TODO(#926): Is this necessary?
      sourceEditor.setBreakpoints(debuggerState.breakpoints.value);
      debuggerState.breakpoints.addListener(() {
        sourceEditor.setBreakpoints(debuggerState.breakpoints.value);
      });
  
      void updateFrames() async {
        if (debuggerState.isPaused.value) {
          // Check for async causal frames; fall back to using regular sync frames.
  
negative.code:         callStackView.showFrames(frames, selectTop: true);
        } else {
          callStackView.clearFrames();
          sourceEditor?.clearExecutionPoint();
        }
      }
  
  