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

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index 173fdefc00c4081cf887d3f47e6628af542f47a6..082fd898cc9b3afdef183abed5c2d7dc4cb9be83 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -4,11 +4,12 @@
 
 class DartBackend extends Backend {
   final List<CompilerTask> tasks;
+  final bool minify;
 
   Map<Element, TreeElements> get resolvedElements =>
       compiler.enqueuer.resolution.resolvedElements;
 
-  DartBackend(Compiler compiler)
+  DartBackend(Compiler compiler, this.minify)
       : tasks = <CompilerTask>[],
       super(compiler);
 
@@ -101,7 +102,7 @@ class DartBackend extends Backend {
     // Create renames.
     Map<Node, String> renames = new Map<Node, String>();
     Map<LibraryElement, String> imports = new Map<LibraryElement, String>();
-    renamePlaceholders(compiler, collector, renames, imports);
+    renamePlaceholders(compiler, collector, renames, imports, minify);
 
     // Sort elements.
     final sortedTopLevels = sortElements(topLevelElements);