Use better names for captured variables in closures.

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

Index: pkg/compiler/lib/src/js_backend/minify_namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/minify_namer.dart b/pkg/compiler/lib/src/js_backend/minify_namer.dart
index b781bc0ad0eb9a8155c638cb272e0a126a786f64..0ba2e4e453e684116b0ae8d0b5e1b9482c70fee0 100644
--- a/pkg/compiler/lib/src/js_backend/minify_namer.dart
+++ b/pkg/compiler/lib/src/js_backend/minify_namer.dart
@@ -34,19 +34,17 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
   /// minified names will always avoid clashing with annotated names or natives.
   @override
   String _generateFreshStringForName(String proposedName,
-      Set<String> usedNames,
-      Map<String, String> suggestedNames,
+      NamingScope scope,
       {bool sanitizeForNatives: false,
        bool sanitizeForAnnotations: false}) {
     String freshName;
-    String suggestion = suggestedNames[proposedName];
-    if (suggestion != null && !usedNames.contains(suggestion)) {
+    String suggestion = scope.suggestName(proposedName);
+    if (suggestion != null && scope.isUnused(suggestion)) {
       freshName = suggestion;
     } else {
-      freshName = _getUnusedName(proposedName, usedNames,
-          suggestedNames.values);
+      freshName = _getUnusedName(proposedName, scope);
     }
-    usedNames.add(freshName);
+    scope.registerUse(freshName);
     return freshName;
   }
 
@@ -85,12 +83,14 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
         String disambiguatedName = name;
         reservePublicMemberName(name, disambiguatedName);
       }
-      usedInstanceNames.add(name);
+      instanceScope.registerUse(name);
       // Getter and setter names are autogenerated by prepending 'g' and 's' to
       // field names.  Therefore there are some field names we don't want to
       // use.  It is implicit in the next line that the banned prefix is
       // only one character.
-      if (_hasBannedPrefix(name)) usedInstanceNames.add(name.substring(1));
+      if (_hasBannedPrefix(name)) {
+        instanceScope.registerUse(name.substring(1));
+      }
     }
 
     // These popular names are present in most programs and deserve
@@ -99,8 +99,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
     // minifier was less stable from version to version of the program being
     // minified.
     _populateSuggestedNames(
-        suggestedInstanceNames,
-        usedInstanceNames,
+        instanceScope,
         const <String>[
             r'$add', r'add$1', r'$and',
             r'$or',
@@ -115,8 +114,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
             r'$tdiv', r'toString$0']);
 
     _populateSuggestedNames(
-        suggestedGlobalNames,
-        usedGlobalNames,
+        globalScope,
         const <String>[
             r'Object', 'wrapException', r'$eq', r'S', r'ioore',
             r'UnsupportedError$', r'length', r'$sub',
@@ -135,8 +133,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
             ]);
   }
 
-  void _populateSuggestedNames(Map<String, String> suggestionMap,
-                               Set<String> used,
+  void _populateSuggestedNames(NamingScope scope,
                                List<String> suggestions) {
     int c = $a - 1;
     String letter;
@@ -145,9 +142,9 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
         assert(c != $Z);
         c = (c == $z) ? $A : c + 1;
         letter = new String.fromCharCodes([c]);
-      } while (_hasBannedPrefix(letter) || used.contains(letter));
-      assert(suggestionMap[name] == null);
-      suggestionMap[name] = letter;
+      } while (_hasBannedPrefix(letter) || scope.isUsed(letter));
+      assert(!scope.hasSuggestion(name));
+      scope.addSuggestion(name, letter);
     }
   }
 
@@ -156,8 +153,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
   // is slightly less efficient than just getting the next name in a series,
   // but it means that small changes in the input program will give smallish
   // changes in the output, which can be useful for diffing etc.
-  String _getUnusedName(String proposedName, Set<String> usedNames,
-                        Iterable<String> suggestions) {
+  String _getUnusedName(String proposedName, NamingScope scope) {
     int hash = _calculateHash(proposedName);
     // Avoid very small hashes that won't try many names.
     hash = hash < 1000 ? hash * 314159 : hash;  // Yes, it's prime.
@@ -177,10 +173,10 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
           h2 ~/= ALPHANUMERIC_CHARACTERS;
         }
         final candidate = new String.fromCharCodes(codes);
-        if (!usedNames.contains(candidate) &&
+        if (scope.isUnused(candidate) &&
             !jsReserved.contains(candidate) &&
             !_hasBannedPrefix(candidate) &&
-            (n != 1 || !suggestions.contains(candidate))) {
+            (n != 1 || scope.isSuggestion(candidate))) {
           return candidate;
         }
         // Try again with a slightly different hash.  After around 10 turns
@@ -188,7 +184,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
         h ~/= 7;
       }
     }
-    return _badName(hash, usedNames);
+    return _badName(hash, scope);
   }
 
   /// Instance members starting with g and s are reserved for getters and
@@ -217,7 +213,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
 
   /// 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 _badName(int hash, NamingScope scope) {
     int count = _badNames.putIfAbsent(hash, () => 0);
     String startLetter =
         new String.fromCharCodes([_letterNumber(hash + count)]);
@@ -226,7 +222,7 @@ class MinifyNamer extends Namer with _MinifiedFieldNamer,
     int i = 0;
     do {
       name = "$startLetter${i++}";
-    } while (usedNames.contains(name));
+    } while (scope.isUsed(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.