Make the fields in Document read-only

pkgs/markdown/CHANGELOG.md

diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index 5e9be55..bff8268 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md
@@ -4,6 +4,7 @@
   the `Document` field, `refLinks`, has been renamed `linkReferences`.
 * **Breaking change:** Remove the deprecated `ExtensionSet.gitHub` field.
   Use `ExtensionSet.gitHubFlavored` instead.
+* **Breaking change:** Make all of the fields on `Document` read-only.
 * Overhaul support for emphasis (`*foo*` and `_foo_`) and strong emphasis
   (`**foo**` and `__foo__`), dramatically improving CommonMark compliance.
 * Improve support for tab characters, and horizontal rules.

pkgs/markdown/lib/src/document.dart

diff --git a/pkgs/markdown/lib/src/document.dart b/pkgs/markdown/lib/src/document.dart
index eb7afd6..7aee5da 100644
--- a/pkgs/markdown/lib/src/document.dart
+++ b/pkgs/markdown/lib/src/document.dart
@@ -9,24 +9,27 @@
 
 /// Maintains the context needed to parse a Markdown document.
 class Document {
-  final Map<String, LinkReference> linkReferences = {};
-  Iterable<BlockSyntax> blockSyntaxes;
-  Iterable<InlineSyntax> inlineSyntaxes;
-  ExtensionSet extensionSet;
-  Resolver linkResolver;
-  Resolver imageLinkResolver;
+  final Map<String, LinkReference> linkReferences = <String, LinkReference>{};
+  final ExtensionSet extensionSet;
+  final Resolver linkResolver;
+  final Resolver imageLinkResolver;
+  final _blockSyntaxes = new Set<BlockSyntax>();
+  final _inlineSyntaxes = new Set<InlineSyntax>();
+
+  Iterable<BlockSyntax> get blockSyntaxes => _blockSyntaxes;
+  Iterable<InlineSyntax> get inlineSyntaxes => _inlineSyntaxes;
 
   Document(
       {Iterable<BlockSyntax> blockSyntaxes,
       Iterable<InlineSyntax> inlineSyntaxes,
       ExtensionSet extensionSet,
       this.linkResolver,
-      this.imageLinkResolver}) {
-    this.extensionSet = extensionSet ?? ExtensionSet.commonMark;
-    this.blockSyntaxes = new Set()
+      this.imageLinkResolver})
+      : this.extensionSet = extensionSet ?? ExtensionSet.commonMark {
+    this._blockSyntaxes
       ..addAll(blockSyntaxes ?? [])
       ..addAll(this.extensionSet.blockSyntaxes);
-    this.inlineSyntaxes = new Set()
+    this._inlineSyntaxes
       ..addAll(inlineSyntaxes ?? [])
       ..addAll(this.extensionSet.inlineSyntaxes);
   }