dart2js: Avoid common prefix for frequent bad names.

pkg/compiler/lib/src/js_backend/minify_namer.dart

 // Copyright (c) 2011, 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_backend;
 
 /**
  * Assigns JavaScript identifiers to Dart variables, class-names and members.
  */
 class MinifyNamer extends Namer {
@@ skipping 191 matching lines @@
       h += name.codeUnitAt(i);
       h &= 0xffffffff;
       h += h << 10;
       h &= 0xffffffff;
       h ^= h >> 6;
       h &= 0xffffffff;
     }
     return h;
   }
 
+  /// Remember bad hashes to avoid using a the same character with long numbers
+  /// for frequent hashes. For example, `closure` is a very common name.
+  Map<int, int> _badNames = new Map<int, int>();
+
   /// If we can't find a hash based name in the three-letter space, then base
   /// the name on a letter and a counter.
   String _badName(int hash, Set<String> usedNames) {
-    String startLetter = new String.fromCharCodes([_letterNumber(hash)]);
+    int count = _badNames.putIfAbsent(hash, () => 0);
+    String startLetter =
+        new String.fromCharCodes([_letterNumber(hash + count)]);
+    _badNames[hash] = count + 1;
     String name;
     int i = 0;
     do {
       name = "$startLetter${i++}";
     } while (usedNames.contains(name));
     // We don't need to check for banned prefix because the name is in the form
     // xnnn, where nnn is a number.  There can be no getter or setter called
     // gnnn since that would imply a numeric field name.
     return name;
   }
@@ skipping 209 matching lines @@
   _BoxFieldNamingScope(Local box, _FieldNamingRegistry registry) :
     super.rootScope(box, registry);
 
   bool containsField(_) => true;
 
   String operator[](Element field) {
     if (!names.containsKey(field)) add(field);
     return names[field];
   }
 }