Add Member.documentationComment and use it to resynthesize documentation from Kernel.

pkg/kernel/lib/binary/ast_to_binary.dart

Index: pkg/kernel/lib/binary/ast_to_binary.dart
diff --git a/pkg/kernel/lib/binary/ast_to_binary.dart b/pkg/kernel/lib/binary/ast_to_binary.dart
index 0462336e05b0bd3a806cfd3a163f2c494ce6672b..16d7f0cc8c9bf01c6e6b3918f0130da4936a5477 100644
--- a/pkg/kernel/lib/binary/ast_to_binary.dart
+++ b/pkg/kernel/lib/binary/ast_to_binary.dart
@@ -415,6 +415,7 @@ class BinaryPrinter extends Visitor {
     Class parent = node.parent;
     writeUInt30(parent.binaryOffset);
     writeName(node.name ?? _emptyName);
+    writeStringReference(node.documentationComment ?? '');
     writeAnnotationList(node.annotations);
     assert(node.function.typeParameters.isEmpty);
     writeNode(node.function);
@@ -444,6 +445,7 @@ class BinaryPrinter extends Visitor {
     }
     writeName(node.name ?? '');
     writeUriReference(node.fileUri ?? '');
+    writeStringReference(node.documentationComment ?? '');
     writeAnnotationList(node.annotations);
     writeOptionalNode(node.function);
     _variableIndexer = null;
@@ -467,6 +469,7 @@ class BinaryPrinter extends Visitor {
     }
     writeName(node.name);
     writeUriReference(node.fileUri ?? '');
+    writeStringReference(node.documentationComment ?? '');
     writeAnnotationList(node.annotations);
     writeNode(node.type);
     writeOptionalNode(node.initializer);
@@ -1370,11 +1373,29 @@ class StringIndexer extends RecursiveVisitor<Null> {
     node.visitChildren(this);
   }
 
+  @override
+  visitConstructor(Constructor node) {
+    putOptional(node.documentationComment);
+    super.visitConstructor(node);
+  }
+
+  @override
+  visitField(Field node) {
+    putOptional(node.documentationComment);
+    super.visitField(node);
+  }
+
   visitNamedExpression(NamedExpression node) {
     put(node.name);
     node.visitChildren(this);
   }
 
+  @override
+  visitProcedure(Procedure node) {
+    putOptional(node.documentationComment);
+    super.visitProcedure(node);
+  }
+
   visitStringLiteral(StringLiteral node) {
     put(node.value);
   }