Roll engine to ed303c628fe4b322529f8cf01ecb38135a2bab73 (dart roll)

Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```

Add consts

bin/internal/engine.version

@@ -1 +1 @@
-8cd27273351ea6e2fef0b3801fe759a8fbe4d162
+ed303c628fe4b322529f8cf01ecb38135a2bab73

dev/manual_tests/lib/card_collection.dart

@@ -344,7 +344,7 @@ class CardCollectionState extends State<CardCollection> {
   }
 
   Shader _createShader(Rect bounds) {
-    return new LinearGradient(
+    return const LinearGradient(
         begin: Alignment.topCenter,
         end: Alignment.bottomCenter,
         colors: const <Color>[const Color(0x00FFFFFF), const Color(0xFFFFFFFF)],

examples/flutter_gallery/lib/demo/cupertino/cupertino_refresh_demo.dart

@@ -152,9 +152,9 @@ class _ListItem extends StatelessWidget {
           new Container(
             width: 38.0,
             child: called
-                ? new Align(
+                ? const Align(
                     alignment: Alignment.topCenter,
-                    child: new Icon(
+                    child: const Icon(
                       CupertinoIcons.phone_solid,
                       color: CupertinoColors.inactiveGray,
                       size: 18.0,
@@ -207,9 +207,9 @@ class _ListItem extends StatelessWidget {
                       letterSpacing: -0.41,
                     ),
                   ),
-                  new Padding(
+                  const Padding(
                     padding: const EdgeInsets.only(left: 9.0),
-                    child: new Icon(
+                    child: const Icon(
                       CupertinoIcons.info,
                       color: CupertinoColors.activeBlue
                     ),

examples/flutter_gallery/lib/demo/cupertino/cupertino_switch_demo.dart

@@ -42,8 +42,8 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
               ],
             ),
             new Column(
-              children: <Widget>[
-                new CupertinoSwitch(
+              children: const <Widget>[
+                const CupertinoSwitch(
                   value: true,
                   onChanged: null,
                 ),
@@ -53,8 +53,8 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
               ],
             ),
             new Column(
-              children: <Widget>[
-                new CupertinoSwitch(
+              children: const <Widget>[
+                const CupertinoSwitch(
                   value: false,
                   onChanged: null,
                 ),

examples/flutter_gallery/lib/demo/video_demo.dart

@@ -182,13 +182,13 @@ class _VideoPlayPauseState extends State<VideoPlayPause> {
               return;
             }
             if (controller.value.isPlaying) {
-              imageFadeAnimation = new FadeAnimation(
-                child: new Icon(Icons.pause, size: 100.0),
+              imageFadeAnimation = const FadeAnimation(
+                child: const Icon(Icons.pause, size: 100.0),
               );
               controller.pause();
             } else {
-              imageFadeAnimation = new FadeAnimation(
-                child: new Icon(Icons.play_arrow, size: 100.0),
+              imageFadeAnimation = const FadeAnimation(
+                child: const Icon(Icons.play_arrow, size: 100.0),
               );
               controller.play();
             }

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

@@ -204,7 +204,7 @@ class _DayPickerGridDelegate extends SliverGridDelegate {
 
   @override
   SliverGridLayout getLayout(SliverConstraints constraints) {
-    final int columnCount = DateTime.daysPerWeek;
+    const int columnCount = DateTime.daysPerWeek;
     final double tileWidth = constraints.crossAxisExtent / columnCount;
     final double tileHeight = math.min(_kDayPickerRowHeight, constraints.viewportMainAxisExtent / (_kMaxDayPickerRowCount + 1));
     return new SliverGridRegularTileLayout(