Bump Dart SDK to `1.20.0-dev.10.0`. (#6212)

* Bump Dart SDK to `1.20.0-dev.10.0`.

`1.20.0-dev.10.0` corresponds to `1.20-RC0`.

* pubspec.lock cleanup

* check for pubspec.lock existence

* cleaned up conditional remove

bin/cache/dart-sdk.version

-1.20.0-dev.5.0
+1.20.0-dev.10.0

bin/flutter

@@ -41,7 +41,7 @@ DART="$DART_SDK_PATH/bin/dart"
 
     echo Building flutter tool...
     FLUTTER_DIR="$FLUTTER_ROOT/packages/flutter"
-    (cd "$FLUTTER_TOOLS_DIR"; "../../bin/cache/dart-sdk/bin/pub" get --verbosity=error --no-packages-dir)
+    (cd "$FLUTTER_TOOLS_DIR"; rm -f pubspec.lock; "../../bin/cache/dart-sdk/bin/pub" get --verbosity=error --no-packages-dir)
     "$DART" --snapshot="$SNAPSHOT_PATH" --packages="$FLUTTER_TOOLS_DIR/.packages" "$SCRIPT_PATH"
     echo $REVISION > "$STAMP_PATH"
   fi

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

@@ -222,7 +222,8 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   void hitTest(HitTestResult result, Point position) {
     assert(renderView != null);
     renderView.hitTest(result, position: position);
-    super.hitTest(result, position);
+    // This super call is safe since it will be bound to a mixed-in declaration.
+    super.hitTest(result, position); //ignore: abstract_super_member_reference
   }
 
   void _forceRepaint() {

packages/flutter_tools/lib/src/dart/analysis.dart

@@ -5,6 +5,7 @@
 import 'dart:collection';
 import 'dart:io';
 
+import 'package:analyzer/error/error.dart';
 import 'package:analyzer/file_system/file_system.dart' as file_system;
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/plugin/options.dart';
@@ -14,7 +15,6 @@ import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/context/builder.dart'; // ignore: implementation_imports
 import 'package:analyzer/src/dart/sdk/sdk.dart'; // ignore: implementation_imports
 import 'package:analyzer/src/generated/engine.dart'; // ignore: implementation_imports
-import 'package:analyzer/src/generated/error.dart'; // ignore: implementation_imports
 import 'package:analyzer/src/generated/java_io.dart'; // ignore: implementation_imports
 import 'package:analyzer/src/generated/source.dart'; // ignore: implementation_imports
 import 'package:analyzer/src/generated/source_io.dart'; // ignore: implementation_imports

packages/flutter_tools/lib/src/dart/summary.dart

@@ -72,11 +72,9 @@ void buildSkyEngineSdkSummary(
   //
   // Build.
   //
-  SummaryBuildConfig config = new SummaryBuildConfig(strongMode: true);
-  BuilderOutput output =
-      new SummaryBuilder(sources, sdk.context, config).build();
+  List<int> bytes = new SummaryBuilder(sources, sdk.context, true).build();
   String outputPath = pathos.join(skyEnginePath, outBundleName);
-  new io.File(outputPath).writeAsBytesSync(output.sum);
+  new io.File(outputPath).writeAsBytesSync(bytes);
 }
 
 Future<Null> buildUnlinkedForPackages(String flutterPath) async {