Store the covariant keyword flag for parameters and fields into Kernel and resynthesize in analyzer.

pkg/kernel/lib/clone.dart

Index: pkg/kernel/lib/clone.dart
diff --git a/pkg/kernel/lib/clone.dart b/pkg/kernel/lib/clone.dart
index 4f1d6814865c3791f0ad08b94f37f212858698e9..74bf8a87db2e0cc3e8127903e4f604534220a0b0 100644
--- a/pkg/kernel/lib/clone.dart
+++ b/pkg/kernel/lib/clone.dart
@@ -360,6 +360,7 @@ class CloneVisitor extends TreeVisitor {
     return variables[node] = new VariableDeclaration(node.name,
         initializer: cloneOptional(node.initializer),
         type: visitType(node.type),
+        isCovariant: node.isCovariant,
         isFinal: node.isFinal,
         isConst: node.isConst,
         isFieldFormal: node.isFieldFormal);
@@ -396,6 +397,7 @@ class CloneVisitor extends TreeVisitor {
     return new Field(node.name,
         type: visitType(node.type),
         initializer: cloneOptional(node.initializer),
+        isCovariant: node.isCovariant,
         isFinal: node.isFinal,
         isConst: node.isConst,
         isStatic: node.isStatic,