dart2js: Implement frequency based naming.

pkg/js_ast/lib/src/printer.dart

 // Copyright (c) 2012, 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.
 
 part of js_ast;
 
 
+typedef String Renamer(Name);
+
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
   final bool preferSemicolonToNewlineInMinifiedOutput;
+  final Renamer renamerForNames;
 
   JavaScriptPrintingOptions(
       {this.shouldCompressOutput: false,
        this.minifyLocalVariables: false,
-       this.preferSemicolonToNewlineInMinifiedOutput: false});
+       this.preferSemicolonToNewlineInMinifiedOutput: false,
+       this.renamerForNames: identityRenamer});
+
+  static String identityRenamer(Name name) => name.name;
 }
 
 
 /// An environment in which JavaScript printing is done.  Provides emitting of
 /// text and pre- and post-visit callbacks.
 abstract class JavaScriptPrintingContext {
   /// Signals an error.  This should happen only for serious internal errors.
   void error(String message) { throw message; }
 
   /// Adds [string] to the output.
@@ skipping 896 matching lines @@
       String fieldWithQuotes = selectorString.value;
       if (isValidJavaScriptId(fieldWithQuotes)) {
         if (access.receiver is LiteralNumber) out(" ", isWhitespace: true);
         out(".");
         out(fieldWithQuotes.substring(1, fieldWithQuotes.length - 1));
         return;
       }
     } else if (selector is Name) {
       if (access.receiver is LiteralNumber) out(" ", isWhitespace: true);
       out(".");
-      out(selector.name);
+      selector.accept(this);
       return;
     }
     out("[");
     visitNestedExpression(selector, EXPRESSION,
                           newInForInit: false, newAtStatementBegin: false);
     out("]");
   }
 
   @override
   void visitNamedFunction(NamedFunction namedFunction) {
@@ skipping 47 matching lines @@
     out(node.value);
   }
 
   @override
   visitStringConcatenation(StringConcatenation node) {
     node.visitChildren(this);
   }
 
   @override
   visitName(Name node) {
-    out(node.name);
+    out(options.renamerForNames(node));
   }
 
   @override
   visitLiteralNumber(LiteralNumber node) {
     outputNumberWithRequiredWhitespace(node.value);
   }
 
   @override
   void visitLiteralNull(LiteralNull node) {
     out("null");
@@ skipping 450 matching lines @@
     }
   }
 
   EnterExitNode exitNode(JavaScriptPrintingContext context, int position) {
     // Enter must happen before exit.
     addToNode(context, position);
     context.exitNode(node, startPosition, position, closingPosition);
     return parent;
   }
 }