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

lib/compiler/implementation/dart_backend/renamer.dart

Index: lib/compiler/implementation/dart_backend/renamer.dart
diff --git a/lib/compiler/implementation/dart_backend/renamer.dart b/lib/compiler/implementation/dart_backend/renamer.dart
index 959f8d8994c16a1a6b9212754d698a7f79176a5d..d8f71b4cebe703b4283a1fe51a217ea7e3a384c8 100644
--- a/lib/compiler/implementation/dart_backend/renamer.dart
+++ b/lib/compiler/implementation/dart_backend/renamer.dart
@@ -10,7 +10,8 @@ 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>();
@@ -18,8 +19,6 @@ void renamePlaceholders(
   // 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);