add unimpl stubs in web source files

Author: flar

lib/web_ui/lib/src/engine/canvaskit/canvaskit_canvas.dart

@@ -104,6 +104,11 @@ class CanvasKitCanvas implements ui.Canvas {
     _canvas.transform(matrix4);
   }
 
+  @override
+  Float64List getTransform() {
+    throw UnimplementedError('getTransform not implemented on CanvasKit back end');
+  }
+
   @override
   void clipRect(ui.Rect rect,
       {ui.ClipOp clipOp = ui.ClipOp.intersect, bool doAntiAlias = true}) {
@@ -136,6 +141,17 @@ class CanvasKitCanvas implements ui.Canvas {
     _canvas.clipPath(path as CkPath, doAntiAlias);
   }
 
+
+  @override
+  ui.Rect getLocalClipBounds() {
+    throw UnimplementedError('getLocalClipBounds not implemented on CanvasKit back end');
+  }
+
+  @override
+  ui.Rect getDestinationClipBounds() {
+    throw UnimplementedError('getDestinationClipBounds not implemented on CanvasKit back end');
+  }
+
   @override
   void drawColor(ui.Color color, ui.BlendMode blendMode) {
     assert(color != null); // ignore: unnecessary_null_comparison

lib/web_ui/lib/src/engine/html/canvas.dart

@@ -96,6 +96,11 @@ class SurfaceCanvas implements ui.Canvas {
     _canvas.transform(matrix4);
   }
 
+  @override
+  Float64List getTransform() {
+    throw UnimplementedError('getTransform not implemented on HTML back end');
+  }
+
   @override
   void clipRect(ui.Rect rect,
       {ui.ClipOp clipOp = ui.ClipOp.intersect, bool doAntiAlias = true}) {
@@ -132,6 +137,16 @@ class SurfaceCanvas implements ui.Canvas {
     _canvas.clipPath(path, doAntiAlias: doAntiAlias);
   }
 
+  @override
+  ui.Rect getLocalClipBounds() {
+    throw UnimplementedError('getLocalClipBounds not implemented on HTML back end');
+  }
+
+  @override
+  ui.Rect getDestinationClipBounds() {
+    throw UnimplementedError('getDestinationClipBounds not implemented on HTML back end');
+  }
+
   @override
   void drawColor(ui.Color color, ui.BlendMode blendMode) {
     assert(color != null); // ignore: unnecessary_null_comparison