Minor improvements to the render tree dump. (#6831)

Also fixes https://github.com/flutter/flutter/issues/6510

packages/flutter/lib/src/painting/box_painter.dart

@@ -1191,7 +1191,7 @@ class BoxDecoration extends Decoration {
   /// If the method is passed a non-empty string argument, then the output will
   /// span multiple lines, each prefixed by that argument.
   @override
-  String toString([String prefix = '']) {
+  String toString([String prefix = '', String indentPrefix]) {
     List<String> result = <String>[];
     if (backgroundColor != null)
       result.add('${prefix}backgroundColor: $backgroundColor');
@@ -1201,8 +1201,15 @@ class BoxDecoration extends Decoration {
       result.add('${prefix}border: $border');
     if (borderRadius != null)
       result.add('${prefix}borderRadius: $borderRadius');
-    if (boxShadow != null)
-      result.add('${prefix}boxShadow: ${boxShadow.map((BoxShadow shadow) => shadow.toString())}');
+    if (boxShadow != null) {
+      if (indentPrefix != null && boxShadow.length > 1) {
+        result.add('${prefix}boxShadow:');
+        for (BoxShadow shadow in boxShadow)
+          result.add('$indentPrefix$shadow');
+      } else {
+        result.add('${prefix}boxShadow: ${boxShadow.map((BoxShadow shadow) => shadow.toString()).join(", ")}');
+      }
+    }
     if (gradient != null)
       result.add('${prefix}gradient: $gradient');
     if (shape != BoxShape.rectangle)

packages/flutter/lib/src/painting/decoration.dart

@@ -79,8 +79,16 @@ abstract class Decoration {
   /// if it is a [BoxDecoration] with definitely no [BackgroundImage]).
   BoxPainter createBoxPainter([VoidCallback onChanged]);
 
+  /// Returns a string representation of this object.
+  ///
+  /// Every line of the output should be prefixed by `prefix`.
+  ///
+  /// If `indentPrefix` is non-null, then the description can be further split
+  /// into sublines, and each subline should be prefixed with `indentPrefix`
+  /// (rather that `prefix`). This is used, for example, by [BoxDecoration] for
+  /// the otherwise quite verbose [BoxShadow] descriptions.
   @override
-  String toString([String prefix = '']) => '$prefix$runtimeType';
+  String toString([String prefix = '', String indentPrefix ]) => '$prefix$runtimeType';
 }
 
 /// A stateful class that can paint a particular [Decoration].

packages/flutter/lib/src/painting/flutter_logo.dart

@@ -220,7 +220,7 @@ class FlutterLogoDecoration extends Decoration {
   }
 
   @override
-  String toString([String prefix = '']) {
+  String toString([String prefix = '', String prefixIndent ]) {
     final String extra = _inTransition ? ', transition $_position:$_opacity' : '';
     if (swatch == null)
       return '$prefix$runtimeType(null, $style$extra)';

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

@@ -2409,7 +2409,10 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     final String descriptionPrefix = childrenDescription != '' ? '$prefixOtherLines \u2502 ' : '$prefixOtherLines   ';
     List<String> description = <String>[];
     debugFillDescription(description);
-    result += description.map((String description) => "$descriptionPrefix$description\n").join();
+    result += description
+      .expand((String description) => description.split('\n'))
+      .map/*<String>*/((String line) => "$descriptionPrefix$line\n")
+      .join();
     if (childrenDescription == '')
       result += '$prefixOtherLines\n';
     result += childrenDescription;

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

@@ -1298,7 +1298,7 @@ class RenderDecoratedBox extends RenderProxyBox {
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
     description.add('decoration:');
-    description.addAll(_decoration.toString("  ").split('\n'));
+    description.addAll(_decoration.toString('  ', '    ').split('\n'));
     description.add('configuration: $configuration');
   }
 }

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

@@ -2560,7 +2560,19 @@ abstract class Element implements BuildContext {
     }
     if (node != null)
       chain.add('\u22EF');
-    return chain.join(' \u2190 ');
+    String result = '';
+    String line = '';
+    for (String link in chain) {
+      if (line != '')
+        line += ' \u2190 ';
+      if (link.length > 3 && line.length + link.length > 65) {
+        result += '$line\n';
+        line = '         ';
+      }
+      line += link;
+    }
+    result += line;
+    return result;
   }
 
   /// A short, textual description of this element.
@@ -3462,7 +3474,10 @@ abstract class RenderObjectElement extends BuildableElement {
   }
 
   void _debugUpdateRenderObjectOwner() {
-    _renderObject.debugCreator = debugGetCreatorChain(10);
+    assert(() {
+      _renderObject.debugCreator = new _DebugCreator(this);
+      return true;
+    });
   }
 
   @override
@@ -3922,6 +3937,13 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
   }
 }
 
+class _DebugCreator {
+  _DebugCreator(this.element);
+  final RenderObjectElement element;
+  @override
+  String toString() => element.debugGetCreatorChain(12);
+}
+
 void _debugReportException(String context, dynamic exception, StackTrace stack, {
   InformationCollector informationCollector
 }) {