Be smarter about when disabling tree-shaking.

dart/sdk/lib/_internal/compiler/implementation/compiler.dart

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index b51e9f5923e46a809e3716a7e9d61ed9dae3dcc0..c850caa3ae3367d60e30319a08121a9cf29b1a8a 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -182,6 +182,10 @@ abstract class Backend {
     ClassElement classElement = element.getEnclosingClass();
     return classElement == compiler.objectClass;
   }
+
+  void enableMirrors() {}
+
+  void registerStaticUse(Element element, Enqueuer enqueuer) {}
 }
 
 /**
@@ -524,7 +528,7 @@ abstract class Compiler implements DiagnosticListener {
 
   bool get analyzeAll => analyzeAllFlag || compileAll;
 
-  bool get compileAll => mirrorsEnabled;
+  bool get compileAll => false;
 
   bool get disableTypeInference => disableTypeInferenceFlag || mirrorsEnabled;
 
@@ -663,6 +667,7 @@ abstract class Compiler implements DiagnosticListener {
     }
     if (uri == Uri.parse('dart:mirrors')) {
       mirrorSystemClass = library.find(const SourceString('MirrorSystem'));
+      backend.enableMirrors();
       metadataHandler = constantHandler;
     } else if (uri == Uri.parse('dart:_collection-dev')) {
       symbolImplementationClass = library.find(const SourceString('Symbol'));