try use_string_buffers lint (#10034)

.analysis_options

@@ -136,7 +136,7 @@ linter:
     - unnecessary_this
     # - use_rethrow_when_possible # not yet tested
     # - use_setters_to_change_properties # not yet tested
-    # - use_string_buffers # not yet tested
+    # - use_string_buffers # https://github.com/dart-lang/linter/pull/664
     # - use_to_and_as_if_applicable # not yet tested
 
     # === pub rules ===

.analysis_options_repo

@@ -134,7 +134,7 @@ linter:
     - unnecessary_this
     # - use_rethrow_when_possible # not yet tested
     # - use_setters_to_change_properties # not yet tested
-    # - use_string_buffers # not yet tested
+    # - use_string_buffers # https://github.com/dart-lang/linter/pull/664
     # - use_to_and_as_if_applicable # not yet tested
 
     # === pub rules ===

packages/flutter/lib/src/rendering/layer.dart

@@ -349,19 +349,21 @@ class ContainerLayer extends Layer {
   String debugDescribeChildren(String prefix) {
     if (firstChild == null)
       return '';
-    String result = '$prefix \u2502\n';
+    final StringBuffer result = new StringBuffer()
+      ..write(prefix)
+      ..write(' \u2502\n');
     Layer child = firstChild;
     int count = 1;
     while (child != lastChild) {
-      result += '${child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502")}';
+      result.write(child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502"));
       count += 1;
       child = child.nextSibling;
     }
     if (child != null) {
       assert(child == lastChild);
-      result += '${child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  ")}';
+      result.write(child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  "));
     }
-    return result;
+    return result.toString();
   }
 }
 

packages/flutter/lib/src/rendering/object.dart

@@ -3059,20 +3059,22 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
   @override
   String debugDescribeChildren(String prefix) {
     if (firstChild != null) {
-      String result = '$prefix \u2502\n';
+      final StringBuffer result = new StringBuffer()
+        ..write(prefix)
+        ..write(' \u2502\n');
       ChildType child = firstChild;
       int count = 1;
       while (child != lastChild) {
-        result += '${child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502")}';
+        result.write(child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502"));
         count += 1;
         final ParentDataType childParentData = child.parentData;
         child = childParentData.nextSibling;
       }
       if (child != null) {
         assert(child == lastChild);
-        result += '${child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  ")}';
+        result.write(child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  "));
       }
-      return result;
+      return result.toString();
     }
     return '';
   }

packages/flutter/lib/src/rendering/semantics.dart

@@ -590,15 +590,18 @@ class SemanticsNode extends AbstractNode {
 
   /// Returns a string representation of this node and its descendants.
   String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {
-    String result = '$prefixLineOne$this\n';
+    final StringBuffer result = new StringBuffer()
+      ..write(prefixLineOne)
+      ..write(this)
+      ..write('\n');
     if (_children != null && _children.isNotEmpty) {
       for (int index = 0; index < _children.length - 1; index += 1) {
         final SemanticsNode child = _children[index];
-        result += '${child.toStringDeep("$prefixOtherLines \u251C", "$prefixOtherLines \u2502")}';
+        result.write(child.toStringDeep("$prefixOtherLines \u251C", "$prefixOtherLines \u2502"));
       }
-      result += '${_children.last.toStringDeep("$prefixOtherLines \u2514", "$prefixOtherLines  ")}';
+      result.write(_children.last.toStringDeep("$prefixOtherLines \u2514", "$prefixOtherLines  "));
     }
-    return result;
+    return result.toString();
   }
 }
 

packages/flutter/lib/src/rendering/viewport.dart

@@ -476,16 +476,18 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
     if (firstChild == null)
       return '$prefix\n';
     int count = indexOfFirstChild;
-    String result = '$prefix \u2502\n';
+    final StringBuffer result = new StringBuffer()
+      ..write(prefix)
+      ..write(' \u2502\n');
     RenderSliver child = firstChild;
     while (child != lastChild) {
-      result += '${child.toStringDeep("$prefix \u251C\u2500${labelForChild(count)}: ", "$prefix \u2502")}';
+      result.write(child.toStringDeep("$prefix \u251C\u2500${labelForChild(count)}: ", "$prefix \u2502"));
       count += 1;
       child = childAfter(child);
     }
     assert(child == lastChild);
-    result += '${child.toStringDeep("$prefix \u2514\u2500${labelForChild(count)}: ", "$prefix  ")}';
-    return result;
+    result.write(child.toStringDeep("$prefix \u2514\u2500${labelForChild(count)}: ", "$prefix  "));
+    return result.toString();
   }
 
   // API TO BE IMPLEMENTED BY SUBCLASSES

packages/flutter/lib/src/widgets/framework.dart

@@ -2936,16 +2936,19 @@ abstract class Element implements BuildContext {
 
   /// A detailed, textual description of this element, includings its children.
   String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {
-    String result = '$prefixLineOne$this\n';
+    final StringBuffer result = new StringBuffer()
+      ..write(prefixLineOne)
+      ..write(this)
+      ..write('\n');
     final List<Element> children = <Element>[];
     visitChildren(children.add);
     if (children.isNotEmpty) {
       final Element last = children.removeLast();
       for (Element child in children)
-        result += '${child.toStringDeep("$prefixOtherLines\u251C", "$prefixOtherLines\u2502")}';
-      result += '${last.toStringDeep("$prefixOtherLines\u2514", "$prefixOtherLines ")}';
+        result.write(child.toStringDeep("$prefixOtherLines\u251C", "$prefixOtherLines\u2502"));
+      result.write(last.toStringDeep("$prefixOtherLines\u2514", "$prefixOtherLines "));
     }
-    return result;
+    return result.toString();
   }
 
   /// Returns true if the element has been marked as needing rebuilding.

packages/flutter/lib/src/widgets/overlay.dart

@@ -560,26 +560,31 @@ class _RenderTheatre extends RenderBox
 
   @override
   String debugDescribeChildren(String prefix) {
-    String result = '';
+    final StringBuffer result = new StringBuffer();
     if (child != null)
-      result += '$prefix \u2502\n${child.toStringDeep('$prefix \u251C\u2500onstage: ', '$prefix \u254E')}';
+      result
+        ..write(prefix)
+        ..write(' \u2502\n')
+        ..write(child.toStringDeep('$prefix \u251C\u2500onstage: ', '$prefix \u254E'));
     if (firstChild != null) {
       RenderBox child = firstChild;
       int count = 1;
       while (child != lastChild) {
-        result += '${child.toStringDeep("$prefix \u254E\u254Coffstage $count: ", "$prefix \u254E")}';
+        result.write(child.toStringDeep("$prefix \u254E\u254Coffstage $count: ", "$prefix \u254E"));
         count += 1;
         final StackParentData childParentData = child.parentData;
         child = childParentData.nextSibling;
       }
       if (child != null) {
         assert(child == lastChild);
-        result += '${child.toStringDeep("$prefix \u2514\u254Coffstage $count: ", "$prefix  ")}';
+        result.write(child.toStringDeep("$prefix \u2514\u254Coffstage $count: ", "$prefix  "));
       }
     } else {
-      result += '$prefix \u2514\u254Cno offstage children';
+      result
+        ..write(prefix)
+        ..write(' \u2514\u254Cno offstage children');
     }
-    return result;
+    return result.toString();
   }
 
   @override