cleanup patch generation to preseve comments and remove @patch

tool/patch_sdk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Command line tool to merge the SDK libraries and our patch files.
 /// This is currently designed as an offline tool, but we could automate it.
 library ddc.tool.patch_sdk;
 
 import 'dart:io';
@@ skipping 153 matching lines @@
   }
 
   void _maybePatch(AstNode node) {
     var externalKeyword = (node as dynamic).externalKeyword;
     if (externalKeyword == null) return;
 
     var name = _qualifiedName(node);
     var patchNode = patches[name];
     if (patchNode == null) throw 'patch not found for $name: $node';
 
-    var code = patchCode.substring(patchNode.offset, patchNode.end);
+    Annotation patch = patchNode.metadata.lastWhere(_isPatchAnnotation);
+    var code = patchCode.substring(patch.endToken.next.offset, patchNode.end);
 
     // For some node like static fields, the node's offset doesn't include
-    // the external keyword.
-    var begin = math.min(node.offset, externalKeyword.offset);
-    edits.replace(begin, node.end, code);
+    // the external keyword. Also starting from the keyword lets us preserve
+    // documentation comments.
+    edits.replace(externalKeyword.offset, node.end, code);
   }
 }
 
 class PatchFinder extends RecursiveAstVisitor {
   final Map patches = <String, Declaration>{};
 
   @override visitFunctionDeclaration(FunctionDeclaration node) {
     _maybeStorePatch(node);
     return super.visitFunctionDeclaration(node);
   }
@@ skipping 122 matching lines @@
   int get length => end - begin;
 
   String toString() => '(Edit @ $begin,$end: "$replace")';
 
   int compareTo(_StringEdit other) {
     int diff = begin - other.begin;
     if (diff != 0) return diff;
     return end - other.end;
   }
 }