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

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 244 matching lines @@
   List<Procedure> procedures;
 }
 
 abstract type Member extends Node {}
 
 type Field extends Member {
   Byte tag = 4;
   CanonicalNameReference canonicalName;
   FileOffset fileOffset;
   FileOffset fileEndOffset;
-  Byte flags (isFinal, isConst, isStatic);
+  Byte flags (isFinal, isConst, isStatic, isCovariant);
   Name name;
   // An absolute path URI to the .dart file from which the field was created.
   UriReference fileUri;
   StringReference documentationComment;
   List<Expression> annotations;
   DartType type;
   Option<Expression> initializer;
 }
 
 type Constructor extends Member {
@@ skipping 649 matching lines @@
 
 type VariableDeclaration {
   // The offset for the variable declaration, i.e. the offset of the start of
   // the declaration.
   FileOffset fileOffset;
 
   // The offset for the equal sign in the declaration (if it contains one).
   // If it does not contain one this should be -1.
   FileOffset fileEqualsOffset;
 
-  Byte flags (isFinal, isConst);
+  Byte flags (isFinal, isConst, isCovariant);
   // For named parameters, this is the parameter name.
   // For other variables, the name is cosmetic, may be empty,
   // and is not necessarily unique.
   StringReference name;
   DartType type;
 
   // For statements and for-loops, this is the initial value.
   // For optional parameters, this is the default value (if given).
   // In all other contexts, it must be Nothing.
   Option<Expression> initializer;
@@ skipping 91 matching lines @@
   Option<DartType> bound;
 }
 
 type TypeParameter {
   // Note: there is no tag on TypeParameter
   StringReference name; // Cosmetic, may be empty, not unique.
   DartType bound; // 'dynamic' if no explicit bound was given.
 }
 
 ```