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

lib/compiler/implementation/dart_backend/backend.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.
 
 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);
 
   void enqueueHelpers(Enqueuer world) { }
   void codegen(WorkItem work) { }
   void processNativeClasses(Enqueuer world,
                             Collection<LibraryElement> libraries) { }
 
   void assembleProgram() {
     /**
@@ skipping 72 matching lines @@
       collector.collect(element, treeElements);
       if (element is ClassElement) {
         classMembers[element].forEach(makePlaceholders);
       }
     }
     topLevelElements.forEach(makePlaceholders);
 
     // 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);
     final sortedClassMembers = new Map<ClassElement, List<Element>>();
     classMembers.forEach((classElement, members) {
       sortedClassMembers[classElement] = sortElements(members);
     });
 
     final unparser = new Unparser.withRenamer((Node node) => renames[node]);
     compiler.assembledCode = emitCode(
@@ skipping 76 matching lines @@
 
 List<Element> sortElements(Collection<Element> elements) {
   compareElements(e0, e1) {
     int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
     if (result != 0) return result;
     return compareBy((e) => e.position().charOffset)(e0, e1);
   }
 
   return sorted(elements, compareElements);
 }