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

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index 27e00d5b55b98b99cdb32ff7e3557df6ccf56834..4f40c43e3011059dfba6eb0b3d80d0ec6b2e3d43 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -167,7 +167,8 @@ class Compiler implements DiagnosticListener {
             this.enableTypeAssertions = false,
             this.enableUserAssertions = false,
             bool emitJavascript = true,
-            generateSourceMap = true])
+            bool generateSourceMap = true,
+            bool minify = false])
       : libraries = new Map<String, LibraryElement>(),
         world = new World(),
         progress = new Stopwatch.start() {
@@ -184,7 +185,7 @@ class Compiler implements DiagnosticListener {
     typesTask = new ti.TypesTask(this);
     backend = emitJavascript ?
         new js_backend.JavaScriptBackend(this, generateSourceMap) :
-        new dart_backend.DartBackend(this);
+        new dart_backend.DartBackend(this, minify);
     enqueuer = new EnqueueTask(this);
     tasks = [scanner, dietParser, parser, resolver, closureToClassMapper,
              checker, typesTask, constantHandler, enqueuer];