Support customizing API (and hence mock) compiler to minify the output.

lib/compiler/implementation/dart_backend/renamer.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.
 
 /**
  * Renames only top-level elements that would let to ambiguity if not renamed.
  * TODO(smok): Make sure that top-level fields are correctly renamed.
  */
 void renamePlaceholders(
     Compiler compiler,
     PlaceholderCollector placeholderCollector,
     Map<Node, String> renames,
-    Map<LibraryElement, String> imports) {
+    Map<LibraryElement, String> imports,
+    Generator topLevelGenerator) {
   final Map<LibraryElement, Map<String, String>> renamed
       = new Map<LibraryElement, Map<String, String>>();
   final Set<String> usedTopLevelIdentifiers = new Set<String>();
   // TODO(antonm): we should also populate this set with top-level
   // names from core library.
   usedTopLevelIdentifiers.add('main'); // Never rename anything to 'main'.
 
-  Generator topLevelGenerator =
-      true ? conservativeGenerator : new MinifyingGenerator('ABCD').generate;
   String generateUniqueName(name) {
     String newName = topLevelGenerator(
         name, usedTopLevelIdentifiers.contains);
     usedTopLevelIdentifiers.add(newName);
     return newName;
   }
 
   rename(library, originalName) =>
       renamed.putIfAbsent(library, () => <String>{})
           .putIfAbsent(originalName, () => generateUniqueName(originalName));
@@ skipping 88 matching lines @@
     int length = alphabet.length;
     StringBuffer resultBuilder = new StringBuffer();
     while (index >= length) {
       resultBuilder.add(alphabet[index % length]);
       index ~/= length;
     }
     resultBuilder.add(alphabet[index]);
     return resultBuilder.toString();
   }
 }