fixes for null safety dart

.gitignore

@@ -9,3 +9,4 @@ xcuserdata
 
 dart/tests/Speedtest.dart.js.deps
 dart/tests/Speedtest.dart.js.map
+.vscode/launch.json

dart/PatchClass.dart

@@ -22,18 +22,16 @@ part of DiffMatchPatch;
  * Class representing one patch operation.
  */
 class Patch {
-  List<Diff> diffs;
-  int start1;
-  int start2;
+  List<Diff> diffs = List.empty(growable: true);
+  int start1 = 0;
+  int start2 = 0;
   int length1 = 0;
   int length2 = 0;
 
   /**
    * Constructor.  Initializes with an empty list of diffs.
    */
-  Patch() {
-    this.diffs = <Diff>[];
-  }
+  Patch();
 
   /**
    * Emulate GNU diff's format.

dart/tests/Speedtest.dart

@@ -5,10 +5,10 @@ import '../DiffMatchPatch.dart';
 // dart2js -O4 --out=Speedtest.dart.js Speedtest.dart
 
 void launch(Event e) {
-  HtmlElement input1 = document.getElementById('text1');
-  HtmlElement input2 = document.getElementById('text2');
-  String text1 = input1.text;
-  String text2 = input2.text;
+  Element? input1 = document.getElementById('text1');
+  Element? input2 = document.getElementById('text2');
+  String? text1 = input1!.text;
+  String? text2 = input2!.text;
 
   DiffMatchPatch dmp = new DiffMatchPatch();
   dmp.Diff_Timeout = 0.0;
@@ -19,20 +19,18 @@ void launch(Event e) {
   DateTime date_end = new DateTime.now();
 
   var ds = dmp.diff_prettyHtml(d);
-  document.getElementById('outputdiv').setInnerHtml(
-      '$ds<BR>Time: ${date_end.difference(date_start)} (h:mm:ss.mmm)',
+  document.getElementById('outputdiv')!.setInnerHtml('$ds<BR>Time: ${date_end.difference(date_start)} (h:mm:ss.mmm)',
       validator: new TrustedNodeValidator());
 }
 
 void main() {
-  document.getElementById('launch').addEventListener('click', launch);
-  document.getElementById('outputdiv').setInnerHtml('');
+  document.getElementById('launch')!.addEventListener('click', launch);
+  document.getElementById('outputdiv')!.setInnerHtml('');
 }
 
 /// A NodeValidator which allows any contents.
 /// The default validator strips 'style' attributes.
 class TrustedNodeValidator implements NodeValidator {
   bool allowsElement(Element element) => true;
-  bool allowsAttribute(Element element, String attributeName, String value)
-      => true;
+  bool allowsAttribute(Element element, String attributeName, String value) => true;
 }