use isEmpty and isNotEmpty (#7207)

packages/flutter/lib/src/cassowary/expression.dart

@@ -39,7 +39,7 @@ class Expression extends EquationMember {
   Expression asExpression() => this;
 
   @override
-  bool get isConstant => terms.length == 0;
+  bool get isConstant => terms.isEmpty;
 
   @override
   double get value => terms.fold(constant, (double value, Term term) => value + term.value);

packages/flutter/lib/src/cassowary/solver.dart

@@ -773,7 +773,7 @@ class Solver {
   }
 
   Result _dualOptimize() {
-    while (_infeasibleRows.length != 0) {
+    while (_infeasibleRows.isNotEmpty) {
       _Symbol leaving = _infeasibleRows.removeLast();
       _Row row = _rows[leaving];
 

packages/flutter/lib/src/foundation/print.dart

@@ -62,12 +62,12 @@ void _debugPrintTask() {
     _debugPrintStopwatch.reset();
     _debugPrintedCharacters = 0;
   }
-  while (_debugPrintedCharacters < _kDebugPrintCapacity && _debugPrintBuffer.length > 0) {
+  while (_debugPrintedCharacters < _kDebugPrintCapacity && _debugPrintBuffer.isNotEmpty) {
     String line = _debugPrintBuffer.removeFirst();
     _debugPrintedCharacters += line.length; // TODO(ianh): Use the UTF-8 byte length instead
     print(line);
   }
-  if (_debugPrintBuffer.length > 0) {
+  if (_debugPrintBuffer.isNotEmpty) {
     _debugPrintScheduled = true;
     _debugPrintedCharacters = 0;
     new Timer(_kDebugPrintPauseTime, _debugPrintTask);

packages/flutter/lib/src/material/data_table.dart

@@ -259,7 +259,7 @@ class DataTable extends StatelessWidget {
   }) : columns = columns,
        _onlyTextColumn = _initOnlyTextColumn(columns), super(key: key) {
     assert(columns != null);
-    assert(columns.length > 0);
+    assert(columns.isNotEmpty);
     assert(sortColumnIndex == null || (sortColumnIndex >= 0 && sortColumnIndex < columns.length));
     assert(sortAscending != null);
     assert(rows != null);

packages/flutter/lib/src/material/icon_theme_data.dart

@@ -100,7 +100,7 @@ class IconThemeData {
       result.add('opacity: $_opacity');
     if (size != null)
       result.add('size: $size');
-    if (result.length == 0)
+    if (result.isEmpty)
       return '<no theme>';
     return result.join(', ');
   }

packages/flutter/lib/src/material/paginated_data_table.dart

@@ -69,7 +69,7 @@ class PaginatedDataTable extends StatefulWidget {
   }) : super(key: key) {
     assert(header != null);
     assert(columns != null);
-    assert(columns.length > 0);
+    assert(columns.isNotEmpty);
     assert(sortColumnIndex == null || (sortColumnIndex >= 0 && sortColumnIndex < columns.length));
     assert(sortAscending != null);
     assert(rowsPerPage != null);

packages/flutter/lib/src/material/popup_menu.dart

@@ -445,7 +445,7 @@ Future<dynamic/*=T*/> showMenu/*<T>*/({
   int elevation: 8
 }) {
   assert(context != null);
-  assert(items != null && items.length > 0);
+  assert(items != null && items.isNotEmpty);
   return Navigator.push(context, new _PopupMenuRoute<dynamic/*=T*/>(
     position: position,
     items: items,

packages/flutter/lib/src/material/scaffold.dart

@@ -930,7 +930,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     if (!config.resizeToAvoidBottomPadding)
       padding = new EdgeInsets.fromLTRB(padding.left, padding.top, padding.right, 0.0);
 
-    if (_snackBars.length > 0) {
+    if (_snackBars.isNotEmpty) {
       final ModalRoute<dynamic> route = ModalRoute.of(context);
       if (route == null || route.isCurrent) {
         if (_snackBarController.isCompleted && _snackBarTimer == null)

packages/flutter/lib/src/material/tabs.dart

@@ -476,7 +476,7 @@ class TabBarSelection<T> extends StatefulWidget {
     this.onChanged,
     @required this.child
   }) : super(key: key)  {
-    assert(values != null && values.length > 0);
+    assert(values != null && values.isNotEmpty);
     assert(new Set<T>.from(values).length == values.length);
     assert(value == null ? true : values.where((T e) => e == value).length == 1);
     assert(child != null);
@@ -847,7 +847,7 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
 
   @override
   void handleStatusChange(AnimationStatus status) {
-    if (config.labels.length == 0)
+    if (config.labels.isEmpty)
       return;
 
     if (_valueIsChanging && status == AnimationStatus.completed) {
@@ -861,7 +861,7 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
 
   @override
   void handleProgressChange() {
-    if (config.labels.length == 0 || _selection == null)
+    if (config.labels.isEmpty || _selection == null)
       return;
 
     if (_lastSelectedIndex != _selection.index) {

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

@@ -380,7 +380,7 @@ class BoxConstraints extends Constraints {
           affectedFieldsList.add('minHeight');
         if (maxHeight.isNaN)
           affectedFieldsList.add('maxHeight');
-        assert(affectedFieldsList.length > 0);
+        assert(affectedFieldsList.isNotEmpty);
         if (affectedFieldsList.length > 1)
           affectedFieldsList.add('and ${affectedFieldsList.removeLast()}');
         String whichFields = '';

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

@@ -1022,7 +1022,7 @@ class PipelineOwner {
         if (node._needsPaint && node.owner == this)
           PaintingContext.repaintCompositedChild(node);
       }
-      assert(_nodesNeedingPaint.length == 0);
+      assert(_nodesNeedingPaint.isEmpty);
     } finally {
       _debugDoingPaint = false;
       Timeline.finishSync();

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

@@ -492,7 +492,7 @@ class RenderTable extends RenderBox {
     assert(rows == null || children == null);
     assert(defaultColumnWidth != null);
     assert(configuration != null);
-    _columns = columns ?? (children != null && children.length > 0 ? children.first.length : 0);
+    _columns = columns ?? (children != null && children.isNotEmpty ? children.first.length : 0);
     _rows = rows ?? 0;
     _children = new List<RenderBox>()..length = _columns * _rows;
     _columnWidths = columnWidths ?? new HashMap<int, TableColumnWidth>();
@@ -691,10 +691,10 @@ class RenderTable extends RenderBox {
       return;
     assert(columns >= 0);
     // consider the case of a newly empty table
-    if (columns == 0 || cells.length == 0) {
-      assert(cells == null || cells.length == 0);
+    if (columns == 0 || cells.isEmpty) {
+      assert(cells == null || cells.isEmpty);
       _columns = columns;
-      if (_children.length == 0) {
+      if (_children.isEmpty) {
         assert(_rows == 0);
         return;
       }
@@ -757,7 +757,7 @@ class RenderTable extends RenderBox {
         dropChild(oldChild);
     }
     _children.clear();
-    _columns = cells.length > 0 ? cells.first.length : 0;
+    _columns = cells.isNotEmpty ? cells.first.length : 0;
     _rows = 0;
     for (List<RenderBox> row in cells)
       addRow(row);
@@ -1282,7 +1282,7 @@ class RenderTable extends RenderBox {
     super.debugFillDescription(description);
     if (border != null)
       description.add('border: $border');
-    if (_columnWidths.length > 0)
+    if (_columnWidths.isNotEmpty)
       description.add('specified column widths: $_columnWidths');
     description.add('default column width: $defaultColumnWidth');
     description.add('table size: $columns\u00D7$rows');

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

@@ -2365,7 +2365,7 @@ abstract class Element implements BuildContext {
     assert(widget != null);
     assert(depth != null);
     assert(_active);
-    if (_dependencies != null && _dependencies.length > 0) {
+    if (_dependencies != null && _dependencies.isNotEmpty) {
       for (InheritedElement dependency in _dependencies)
         dependency._dependents.remove(this);
       // For expediency, we don't actually clear the list here, even though it's
@@ -2629,7 +2629,7 @@ abstract class Element implements BuildContext {
     String result = '$prefixLineOne$this\n';
     List<Element> children = <Element>[];
     visitChildren(children.add);
-    if (children.length > 0) {
+    if (children.isNotEmpty) {
       Element last = children.removeLast();
       for (Element child in children)
         result += '${child.toStringDeep("$prefixOtherLines\u251C", "$prefixOtherLines\u2502")}';
@@ -2803,7 +2803,7 @@ abstract class BuildableElement extends Element {
 
   @override
   void activate() {
-    final bool hadDependencies = ((_dependencies != null && _dependencies.length > 0) || _hadUnsatisfiedDependencies);
+    final bool hadDependencies = ((_dependencies != null && _dependencies.isNotEmpty) || _hadUnsatisfiedDependencies);
     super.activate(); // clears _dependencies, and sets active to true
     if (_dirty) {
       if (_inDirtyList) {

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

@@ -432,7 +432,7 @@ class _HeroParty {
   }
 
   void setAnimation(Animation<double> animation) {
-    assert(animation != null || _heroes.length == 0);
+    assert(animation != null || _heroes.isEmpty);
     if (animation != _currentAnimation) {
       _clearCurrentAnimation();
       _currentAnimation = animation;

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

@@ -644,7 +644,7 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
   /// The only route that cannot be popped off the navigator is the initial
   /// route.
   bool canPop() {
-    assert(_history.length > 0);
+    assert(_history.isNotEmpty);
     return _history.length > 1 || _history[0].willHandlePopInternally;
   }
 

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

@@ -319,7 +319,7 @@ abstract class LocalHistoryRoute<T> extends Route<T> {
 
   @override
   bool didPop(T result) {
-    if (_localHistory != null && _localHistory.length > 0) {
+    if (_localHistory != null && _localHistory.isNotEmpty) {
       LocalHistoryEntry entry = _localHistory.removeLast();
       assert(entry._owner == this);
       entry._owner = null;
@@ -331,7 +331,7 @@ abstract class LocalHistoryRoute<T> extends Route<T> {
 
   @override
   bool get willHandlePopInternally {
-    return _localHistory != null && _localHistory.length > 0;
+    return _localHistory != null && _localHistory.isNotEmpty;
   }
 }
 

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

@@ -59,7 +59,7 @@ class TableRow {
       result.write('$decoration, ');
     if (children == null) {
       result.write('child list is null');
-    } else if (children.length == 0) {
+    } else if (children.isEmpty) {
       result.write('no children');
     } else {
       result.write('$children');
@@ -137,7 +137,7 @@ class Table extends RenderObjectWidget {
       return true;
     });
     assert(() {
-      if (children.length > 0) {
+      if (children.isNotEmpty) {
         final int cellCount = children.first.children.length;
         if (children.any((TableRow row) => row.children.length != cellCount)) {
           throw new FlutterError(
@@ -195,7 +195,7 @@ class Table extends RenderObjectWidget {
   @override
   RenderTable createRenderObject(BuildContext context) {
     return new RenderTable(
-      columns: children.length > 0 ? children[0].children.length : 0,
+      columns: children.isNotEmpty ? children[0].children.length : 0,
       rows: children.length,
       columnWidths: columnWidths,
       defaultColumnWidth: defaultColumnWidth,
@@ -209,7 +209,7 @@ class Table extends RenderObjectWidget {
 
   @override
   void updateRenderObject(BuildContext context, RenderTable renderObject) {
-    assert(renderObject.columns == (children.length > 0 ? children[0].children.length : 0));
+    assert(renderObject.columns == (children.isNotEmpty ? children[0].children.length : 0));
     assert(renderObject.rows == children.length);
     renderObject
       ..columnWidths = columnWidths
@@ -318,7 +318,7 @@ class _TableElement extends RenderObjectElement {
   void _updateRenderObjectChildren() {
     assert(renderObject != null);
     renderObject.setFlatChildren(
-      _children.length > 0 ? _children[0].children.length : 0,
+      _children.isNotEmpty ? _children[0].children.length : 0,
       _children.expand((_TableElementRow row) => row.children.map((Element child) => child.renderObject)).toList()
     );
   }